ref: Drop unused and deprecated ListNewCommits RPC
The ListNewCommits RPC used to list all commits which were returned by
git rev-list $newrev --not --all
, that is all commits which aren't
referenced by any existing ref. This RPC was replaced by ListCommits,
which is a lot more flexible and allows passing an arbitrary number of
revisions, including pseudo-refs "--not" and "--all". Downstream has
been converted in e8a98485fe7 (ff: Remove :list_commits
feature flag,
2021-07-23) to exclusively use ListCommits, so ListNewCommits is now
unused.
Remove ListNewCommits.