chan_sip.c: Clear scheduled immediate events on unload.

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

The reordering of chan_sip's shutdown is to handle any immediate events
that get put onto the scheduler so resources aren't leaked.  The typical
immediate events at this time are going to be concerned with stopping
other scheduled events.

ASTERISK-25023

Change-Id: I3f6540717634f6f2e84d8531a054976f2bbb9d20