Disable unmount option for / or /home

Summary:
This disables the 'Unmount' context menu in the Places panel for discs
corresponding to / and /home.

It does not make much sense to offer an option that will always fail.

Twin of D15989 for Dolphin

BUG: 399659

Reviewers: Frameworks, ngraham

Reviewed By: ngraham

Subscribers: dfaure, broulik, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16141