C++: The editor plugin doesn’t need to track currentEditorChanged

This was done to do a semantic re-highlight, but the CppEditorSupport
now handles this by also listening for currentEditorChanged signals.

Change-Id: Iaca6212f88c8d02203727f5d89d093c328b9e33c
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>