ui/sdl2: remove workaround forcing x11

ui/sdl2: remove workaround forcing x11

This workaround was put in place in the original implementation almost
10 years ago, considering a very old SDL2 version. Currently it prevents
users to run in a wayland-only environment without manually forcing the
backend.
The SDL2 wayland backend has been supported by distributions for a very
long time (e.g. in Fedora, first available 8 years ago), and is now
considered stable and becoming the default for new SDL2 releases.
Instead of requiring the x11 backend to exist by default, let new qemu
releases run with the default chosen by the installed SDL2 version.

Signed-off-by: Erico Nunes <ernunes@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230301141205.514338-1-ernunes@redhat.com>