make hotkey-driven garbage reporting dev-only

direct/src/showbase/GarbageReport.py

@@ -553,6 +553,8 @@ def checkForGarbageLeaks():
     return numGarbage
 
 def b_checkForGarbageLeaks(wantReply=False):
+    if not __dev__:
+        return 0
     # does a garbage collect on the client and the AI
     # returns number of client garbage leaks
     # logs leak info and terminates (if configured to do so)