Revert "Disclaimer message to add in the main README."

This reverts commit d2ae82db

.github/github_disclaimer.md

-# Initial disclaimer
-This project is now tracked on our hosted gitlab server at:
-https://git.duniter.org/clients/cesium/cesium
-
-The current github repository is a simple clone taken up to date at each push on the main gitlab repository.
-
-All contributions should be performed on the main gitlab repository.
-
-Pull requests proposed on github would generate more work for the main contributors.
-
-Issues can be submitted on github. However, all issues created on github will be duplicated on gitlab manually and closed with a link to the gitlab issue.
-
-
-# Original README.md