[KMountPoint] Remove AIX support

Summary:
AIX has not seen a release for 3 years and can be considered legacy
and completely untested. Remove supporting code.

Reviewers: Frameworks, dhaumann

Reviewed By: dhaumann

Subscribers: dhaumann, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15157