Bug#970020: gdm3: System X11 Default shouldn't be shown when X isn't installed

[Wout B]

Package: gdm3
Version: 3.36.2-1
Severity: normal
X-Debbugs-Cc: wbertrums at gmail.com

Dear Maintainer,

When only having a non-GNOME (Wayland) session installed, such as Sway,
GDM will by default choose Debian's "System X11 Default" session. This
results in it immediately going back to the login screen when trying to
login, unless you manually change what session you use. The "System X11
Default" session shouldn't even be shown if there is no X server
installed, and preferably also not if a X server is installed but no X
session is installed (when using Xwayland that'd be the case).

-- System Information:
Debian Release: bullseye/sid
  APT prefers unstable-debug
  APT policy: (500, 'unstable-debug'), (500, 'unstable')
Architecture: amd64 (x86_64)

Kernel: Linux 5.8.0-1-amd64 (SMP w/16 CPU threads)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8),
LANGUAGE=en_US:en
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled