Remove unmaintained and buggy CVS plugin

The plugin causes hangs or crashes of KDevelop during some of the most
 common operations.

We haven't received any external bug reports for
 this (nor from 5.0-beta1 to 5.2.1 when the plugin was entirely
 non-functional), so it appears that no-one has attempted to use this
 feature.

Advertising this as a "feature" when it's unusable with no real prospect
 of being fixed will only hurt KDevelop's reputation if anyone does try
 to use it. Of course the code is in git history in case it's wanted in
 future.