mpt3sas - remove unused fw_event_work elements

Firmware events are queued up using the fw_event_work's struct work, not
its delayed_work member.  The initial driver for SAS2 controllers had
handled firmware reset using the rescan barrier and was later redesigned
through "mpt2sas: [Resend] Host Reset code cleanup".  The delayed_work
variables are now unused and may provoke CONFIG_DEBUG_OBJECTS_TIMERS
"assert_init not available" false warnings in
_scsih_fw_event_cleanup_queue.

Cleanup fw_event_work's unused entries, update its kerneldoc, and
update _scsih_fw_event_cleanup_queue accordingly.

Fixes: 146b16c8

 (mpt3sas: Refcount fw_events and fix unsafe list usage)
Signed-off-by: Joe Lawrence <joe.lawrence@stratus.com>
Acked-by: Chaitra P B <chaitra.basappa@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>