chan_sip.c: Adjust how dialog_unlink_all() stops scheduled events.

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

* Make dialog_unlink_all() unschedule all items at once in the sched
thread.

ASTERISK-25023

Change-Id: I7743072fb228836e8228b72f6dc46c8cc50b3fb4