libweston: deduplicate compositor shutdown in backend_init failure path

If backend initialization fails, weston_compositor_shutdown() is called
twice, once right away in weston_compositor_load_backend(), and once in
weston_compositor_destroy().
Remove the first and fix a segfault when trying to weston_plane_remove()
the primary plane a second time.

Fixes: 90c11cf4 ("libweston: move weston_compositor_shutdown call out of backends")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>