cam: remove sim callout

Nothing is using the sim callout to unfreeze the queue. Remove it to
simplify the SIM. This was introduced in the original CAM commit in 1998
but setting the CAM_SIM_REL_TIMEOUT_PENDING flag was removed in 1999 in
commit 87cfaf0e1fbd which reworked how bus reset worked. That work was
merged just after 3.2R was released. Remove the unused residuals.

Sponsored by: Netflix
Reviewed by: scottl@, mav@
Differential Revision: https://reviews.freebsd.org/D30383