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-wayland.c b/src/compositor-wayland.c
index 57e850f..5f73c78 100644
--- a/src/compositor-wayland.c
+++ b/src/compositor-wayland.c
@@ -1464,8 +1464,7 @@
 	else
 		weston_seat_init_keyboard(&input->base, keymap);
 
-	if (keymap)
-		xkb_keymap_unref(keymap);
+	xkb_keymap_unref(keymap);
 
 	return;