[KMountPoint] Remove Windows CE support

Summary:
The Windows CE line ended in 2012, with Windows Mobile 8. Remove supporting
code.

Reviewers: Frameworks, Windows, vonreth, dhaumann

Reviewed By: Windows, vonreth, dhaumann

Subscribers: dhaumann, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15158