storage: remove confusing code in StorePool
storage: remove confusing code in StorePool

Previously, `storeDetail.status()` had a different definition of
"dead" than `storeDetail.isDead()`. This was presumably done to
avoid adding stores to the list of available stores if we didn't
have a gossiped store descriptor. This is just a slight change
for clarity and to remove a TODO.

Fixes #12204

vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,paperstreet/cockroach,maxlang/cockroach,glycerine/cockroach,vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,petermattis/cockroach,glycerine/cockroach,glycerine/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,glycerine/cockroach,petermattis/cockroach,vaibhavarora/cockroach,glycerine/cockroach,vaibhavarora/cockroach,maxlang/cockroach,paperstreet/cockroach,petermattis/cockroach,paperstreet/cockroach,maxlang/cockroach,maxlang/cockroach,glycerine/cockroach,petermattis/cockroach,vaibhavarora/cockroach,maxlang/cockroach,petermattis/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,maxlang/cockroach,glycerine/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,glycerine/cockroach,petermattis/cockroach,maxlang/cockroach,maxlang/cockroach