ref: Remove deprecated FindAllBranchNames and FindAllTagNames RPCs
We have deprecated the FindAllBranchNames and FindAllTagNames RPCs in favor of ListRefs in commit 55797560 (proto: Deprecate FindAllBranchNames and FindAllTagNames, 2022-12-07). As downstream users have been converted to use the new RPCs already we are thus safe to remove these RPCs.
Closes #4658 (closed).