[ref] rust 2018: remove useless extern crate

bin/durs-server/Cargo.toml

bin/durs-server/src/main.rs

doc/fr/developper-un-module-durs.md

lib/core/conf/keys.rs

lib/core/conf/lib.rs

lib/core/core/change_conf.rs

lib/core/core/cli/dbex.rs

lib/core/core/cli/keys.rs

lib/core/core/cli/mod.rs

lib/core/core/cli/modules.rs

lib/core/core/cli/reset.rs

lib/core/core/cli/start.rs

lib/core/core/lib.rs

lib/core/message/lib.rs

lib/core/module/lib.rs

lib/core/network/cli/sync.rs

lib/core/network/lib.rs

lib/modules/blockchain/blockchain-dal/certs.rs

lib/modules/blockchain/blockchain-dal/lib.rs

lib/modules/blockchain/blockchain-dal/sources.rs