Revert "res_pjsip_session: Release media resources on session end quicker."

This reverts commit 98709642d6.

See the 15 branch review.

Change-Id: I8476b3cdacaad5157fa36b6247d0e4cdf1e8d5c6

res/res_pjsip_session.c

@@ -2554,11 +2554,6 @@ static int session_end(void *vsession)
 			iter->session_end(session);
 		}
 	}
-
-	/* Release any media resources. */
-	ao2_cleanup(session->media);
-	session->media = NULL;
-
 	return 0;
 }