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/screen-share.c b/src/screen-share.c
index 9e81ef9..524a0ca 100644
--- a/src/screen-share.c
+++ b/src/screen-share.c
@@ -214,8 +214,7 @@
 	else
 		weston_seat_init_keyboard(&seat->base, keymap);
 
-	if (keymap)
-		xkb_keymap_unref(keymap);
+	xkb_keymap_unref(keymap);
 
 	return;