chan_sip.c: Fix session timers deadlock potential.

This patch is part of a series to resolve deadlocks in chan_sip.c.

Stopping a scheduled event can result in a deadlock if the scheduled event
is running when you try to stop the event.  If you hold a lock needed by
the scheduled event while trying to stop the scheduled event then a
deadlock can happen.  The general strategy for resolving the deadlock
potential is to push the actual starting and stopping of the scheduled
events off onto the scheduler/do_monitor() thread by scheduling an
immediate one shot scheduled event.  Some restructuring may be needed
because the code may assume that the start/stop of the scheduled events is
immediate.

ASTERISK-25023

Change-Id: I6d65269151ba95e0d8fe4e9e611881cde2ab4900

channels/sip/include/sip.h

@@ -959,7 +959,6 @@ struct sip_st_dlg {
 	int st_cached_max_se;              /*!< Session-Timers cached Session-Expires */
 	enum st_mode st_cached_mode;       /*!< Session-Timers cached M.O. */
 	enum st_refresher st_cached_ref;   /*!< Session-Timers session refresher */
-	unsigned char quit_flag:1;         /*!< Stop trying to lock; just quit */
 };