screenshot: Remove GNOME Shell implementation

GNOME Shell has started restricting access for it's Screenshot D-Bus API
to internal components only [1] for security reasons. In other words,
this will start failing, so remove it in favor of just using the
freedesktop portal, which should always work.

[1]: gnome-shell!1970
8 jobs for !527 with nielsdg/remove-gnome-shell in 34 minutes and 26 seconds (queued for 3 seconds)
detached
Status Name Job ID Coverage
  Prepare
passed build-image #1663265

00:01:11

passed build-image-win64 #1663266

00:01:08

 
  Dependencies
passed deps-debian #1663267

00:01:47

passed deps-win64 #1663268

00:10:29

 
  Gimp
passed gimp-distcheck-debian #1663269

00:26:18

passed gimp-meson-debian #1665147

00:05:06

passed gimp-win64 #1663271

00:07:51

failed gimp-meson-debian #1663270

00:05:06