mpr/mps: Remove write-only flag and callout

The discovery callout is initialized and cancelled only, making it
write-only. Remove a state flag associated with it being pending as well
as two defines that aren't used that are associated with it. Remove
MP?SAS_SHUTDOWN flag, which is unused.

Sponsored by: Netflix
Reviewed by: ken, scottl, mav
Differential Revision: https://reviews.freebsd.org/D33925