C++: clean-up CppCompletionAssist.

This is step 1 of 2 for merging the various provider factories into a
single class. Merging has the advantage that selecting based on editor
(content) mime-type only has to select one class, instead of re-doing
the selection for each class separately.

Change-Id: I11f815151bd4769ae6028b636793d6a80d02e202
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>