More dependency cleanup (#825)

* Removes `rlp` dependency

* Enable warnings about unused crates in the CI

* Remove all the unused crates

* Make tests working again

substrate/srml/support/src/lib.rs

@@ -29,7 +29,6 @@ extern crate serde;
 
 extern crate sr_std as rstd;
 extern crate sr_io as runtime_io;
-extern crate substrate_primitives as primitives;
 extern crate substrate_metadata;
 
 extern crate mashup;