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.