Remove unused functions

Summary:
showMessage() is only declared, but never defined.

includeFolders() and fetchMountPoint(url) are not used.

Depends on D27611

Reviewers: #baloo, ngraham, davidedmundson

Reviewed By: #baloo, ngraham, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27612