Don't protect xkb_*_unref from NULL

Since xkbcommon-0.3.0, which is required by weston, a NULL argument
doesn't do anything.

Signed-off-by: Ran Benita <ran234@gmail.com>
diff --git a/src/compositor-x11.c b/src/compositor-x11.c
index 984d799..3494e34 100644
--- a/src/compositor-x11.c
+++ b/src/compositor-x11.c
@@ -322,8 +322,7 @@
 	keymap = x11_compositor_get_keymap(c);
 	if (weston_seat_init_keyboard(&c->core_seat, keymap) < 0)
 		return -1;
-	if (keymap)
-		xkb_keymap_unref(keymap);
+	xkb_keymap_unref(keymap);
 
 	x11_compositor_setup_xkb(c);