common.h: remove cxfree

Apparently XFree does take NULL.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/common.h

@@ -538,16 +538,6 @@ static inline struct timespec get_time_timespec(void) {
 	return tm;
 }
 
-/**
- * Wrapper of XFree() for convenience.
- *
- * Because a NULL pointer cannot be passed to XFree(), its man page says.
- */
-static inline void cxfree(void *data) {
-	if (data)
-		XFree(data);
-}
-
 /**
  * Return the painting target window.
  */