doc: Remove Windows from pip installation (#289)

Silkaj doesn’t work on Windows: #437

Clean documentation:
- Remove outdated libsodium18 # Debian Stretch
- Remove duplicate python3-dev

doc/install_pip.md

 # Install Silkaj with Pip
 
-You have to use a shell on GNU/Linux or a command tool (`cmd.exe`) on Windows.
+You have to use a shell on Unix system.
 
-## GNU/Linux
-
-The system must use UTF-8 locales…
+## GNU/Linux/BSD
 
 ### Install libsodium
 
 ```bash
 sudo apt install libsodium23 # Debian Buster
-sudo apt install libsodium18 # Debian Stretch
 sudo dnf install libsodium # Fedora
 ```
 
@@ -26,12 +23,6 @@ On Ubuntu (14.04 and 16.04) and Debian 8, you need this package too:
 sudo apt install libffi-dev
 ```
 
-Raspbian and Linux Mint are reported to require the installation of this package too:
-
-```bash
-sudo apt install python3-dev
-```
-
 ### Completing `PATH`
 
 After intallation, if you get a `bash: silkaj: command not found` error, you should add `~/.local/bin` to your `PATH`:
@@ -57,78 +48,14 @@ Then, install Silkaj with:
 pip3 install silkaj --user
 ```
 
-## Windows
-
-### Administrator rights
-
-Please note that the administrator rights might be mandatory for some of these operations.
-
-### The `PATH` variable
-
-The main issue on Windows is about finding where are the requested files.
-
-Python must be installed (version 3.5 minimum). For instance https://sourceforge.net/projects/winpython/
-
-You can test that Python is available by opening a command tool (cmd.exe) and running:
-
-```bash
-C:\>python --version
-Python 3.6.7
-```
-
-When installing Python, take care to specify the good folder (for instance: `C:\WPy-3670`)
-
-After the installation, you commonly have to add by yourself this folder in the `PATH` environment variable:
-
-To make it by command tool (cmd.exe):
-
-```bash
-set PATH=%PATH%;C:\WPy-3670\
-```
-
-Then you have to exit the cmd tool so that `PATH` variable can be updated internally.
-
-You may right click on computer, then go to Advanced System Parameters and use in the bottom the button Environment Variables.
-
-In order to be able to use silkaj and specifically the OpenSSL binaries, you also have to add the following folder to the `PATH` variable:
-C:\\WPy-3670\\python-3.6.7.amd64\\Lib\\site-packages\\PyQt5\\Qt\\bin\\
-
-```bash
-set PATH=%PATH%;C:\WPy-3670\python-3.6.7.amd64\Lib\site-packages\PyQt5\Qt\bin\
-```
-
-### Creating the command file `silkaj.bat`
-
-In order to be able to launch silkaj as a Windows command, you have to create a file `silkaj.bat` in the following folder:
-`C:\WPy-3670\python-3.6.7.amd64\Scripts\silkaj.bat`
-
-containing:
-
-```bash
-rem @echo off
-python "%~dpn0" %*
-```
-
-and then to add this folder into the `PATH` variable:
-
-```bash
-set PATH=%PATH%;C:\WPy-3670\python-3.6.7.amd64\Scripts\
-```
-
-## Install directly from internet (implicitely uses binaries from website Pypi)
+## Install from PyPI
 
-Assuming that Python v3 and pip version 3 are installed and available. You can check with:
+Assuming that Python and `pip` are installed and available. You can check with:
 
 ```bash
 pip3 --version
 ```
 
-### Install for all users
-
-```bash
-pip3 install silkaj
-```
-
 ### Install for current user only
 
 ```bash
@@ -147,57 +74,8 @@ pip3 install silkaj --user --upgrade
 pip3 uninstall silkaj --user
 ```
 
-### Check silkaj is working
-
-```bash
-silkaj info
-```
-
-______________________________________________________________________
-
-## Install from original sources from the forge
-
-### Retrieve silkaj sources on linux
-
-```bash
-sudo apt install git
-git clone https://git.duniter.org/clients/python/silkaj.git
-cd silkaj
-```
-
-### Retrieve silkaj sources on Windows
-
-First, you have to install the git tool from https://git-scm.com/download/win
-
-Then change directory to where you want to download the sources, for instance:
-
-```bash
-cd  ~/appdata/Roaming/
-```
-
-Then download Silkaj sources (`dev` branch) with git tool
-which will create a folder silkaj
-
-```bash
- git clone https://git.duniter.org/clients/python/silkaj.git
-```
-
-Then change directory to the downloaded folder
-
-```bash
- cd ~/appdata/Roaming/silkaj
-```
-
-### Install
-
-After being sure you have changed directory to the downloaded folder
-
-```bash
-pip3 install .
-```
-
-Or install it as "editable", for development:
+### Check Silkaj is working
 
 ```bash
-pip3 install -e .
+silkaj
 ```