Drop restriction on when to emit the keyboard layout changed OSD

Summary:
We had bound this to whether we use libinput as the keyboard layout
change got emitted from within our Xkb class. This class gets used
also on X11 for modifier only shortcuts.

With the latest changes the notification got split out from Xkb into
an own class which is not used on X11 any more. Thus the check is no
longer used.

By removing the check we can test the OSD in a nested setup.

Test Plan: OSD is shown when running a nested Plasma

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D4244