Remove duplicated auto backend resolution

Summary:
kwin_wayland now automatically chooses an appropriate backend, such as
DRM, nested wayland or nested X. If nested it will automatically go into
windowed mode regardless of whether --windowed is set and works fine.

Backend choosing logic existed duplicated in older code for kwin_wayland
--windowed, with the subtle unexpected difference that it preferred
running as an X client over running as a wayland cient if both are
present. This simplifies codes and syncs automatic resolution behaviour.

kwin_wayland --windowed with --x11-display  or --wayland-display will
remain the same.

Test Plan:
Ran kwin_wayland with and without --windowed inside another wayland.
Got the same backend chosen
Tested that kwin_wayland (without --windowed) on an X machine worked just fine.

Reviewers: #kwin, mart

Reviewed By: #kwin, mart

Subscribers: mart, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13658