The `compact_stack()` function is exposed via `reftable_dump_main()`,
which ultimately ends up being wired into "test-tool reftable". It is
never used by our tests though, and nowadays we have wired up support
for stack compaction into git-pack-refs(1).
Remove the code.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>