xwayland: Avoid using epoxy_has_egl()

There's no real point - if we don't have EGL then the extension check is
also going to fail - and the entrypoint is new in 1.5.0, which we don't
need to require yet.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>