main/pbx.c: Fix double lock of contexts lock introduced by r429967

We only need to hold the context_merge_lock once. Locking it twice will make
many other parts of Asterisk very sad.

ASTERISK-24641 #close


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430111 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/pbx.c

@@ -11858,8 +11858,6 @@ static void presence_state_cb(void *unused, struct stasis_subscription *sub, str
 		return;
 	}
 
-	ast_mutex_lock(&context_merge_lock);/* Hold off ast_merge_contexts_and_delete */
-
 	cmpdevice = ast_alloca(sizeof(*cmpdevice) + strlen(presence_state->provider));
 	strcpy(cmpdevice->hintdevice, presence_state->provider);