i3-input: eliminate xcb_change_gc_single usage with C99

i3-input/xcb.c

@@ -20,14 +20,6 @@
 
 #include "i3-input.h"
 
-/*
- * Convenience-wrapper around xcb_change_gc which saves us declaring a variable
- *
- */
-void xcb_change_gc_single(xcb_connection_t *conn, xcb_gcontext_t gc, uint32_t mask, uint32_t value) {
-        xcb_change_gc(conn, gc, mask, &value);
-}
-
 /*
  * Returns the colorpixel to use for the given hex color (think of HTML).
  *