xwayland: Always require X11 authentication

This reverts commit eac227a2.

Currently, Flatpak applications can bypass the X11 permission setting
and access the X server through abstract sockets because X11 authentication
is not enforced for the current user ID.

Fix this by always requiring X11 authentication for Xwayland. This also
means applications without XAUTHORITY set to the file with Mutter's
Xwayland credentials cannot connect to X, including apps launched from
VT or SSH.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2633>