Switch to new X-KDevelop-Languages key

Summary:
Using "Objective-C", as it is the official name, short enough,
not too often to be types, and people seem to not agree whether
ObjC or Obj-C is a good abbr.
Removing entry from kdevcmakedocumentation, as it does not
implement ILanguageSupport.

Reviewers: #kdevelop, mwolff

Reviewed By: #kdevelop, mwolff

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4424