Remove "Full Screen(All Monitors)" when only one monitor exists

Summary: When there is only one screen available the "Current Screen" and "Full Screen (All monitors)" options are identical, so this patch removes the latter.

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Subscribers: ngraham, nicolasfella, #spectacle

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D18609