Demote `org.gnome.SettingsDaemon.XSettings` from required components

Now that gnome-shell/mutter has switched to Xwayland on demand by default (mutter!1673 (merged)), killing repeatedly gsd-xsettings should not cause a fail-whale:

gnome-session-binary: Unrecoverable failure in required component org.gnome.SettingsDaemon.XSettings.desktop
gnome-session: gnome-session-binary: WARNING: App 'org.gnome.SettingsDaemon.XSettings.desktop' respawning too quickly
gnome-session: gnome-session-binary: WARNING: App 'org.gnome.SettingsDaemon.XSettings.desktop' exited with code 1
gsd-xsettings: gsd-xsettings: Fatal IO error 11 (Resource temporarily unavailable) on X server :3.

That means demoting org.gnome.SettingsDaemon.XSettings from required components so that failure to restart gsd-xsettings should not be a fatal error.

On my system, I can reproduce easily:

1. Enable the experimental feature autoclose-xwayland in mutter
2. Start a new GNOME Shell session
3. Start xterm to spawn Xwayland
4. Close xterm, wait for 10 seconds for mutter to kill Xwayland
5. Start xterm to respawn Xwayland again
6. Close xterm, wait for 10 seconds for mutter to kill Xwayland

→ the fail-whale dialog shows up

Note: I can reproduce at will on my system with the steps above, while @carlosg cannot, so maybe there's something wrong on my jhbuild installation, nevertheless gsd-xsettings should not be treated as required (or required) component anymore with Xwayland on demand.