reftable/stack: do not auto-compact twice in `reftable_stack_add()`

reftable/stack: do not auto-compact twice in `reftable_stack_add()`

In 5c086453ff (reftable/stack: perform auto-compaction with
transactional interface, 2023-12-11), we fixed a bug where the
transactional interface to add changes to a reftable stack did not
perform auto-compaction by calling `reftable_stack_auto_compact()` in
`reftable_stack_addition_commit()`. While correct, this change may now
cause us to perform auto-compaction twice in the non-transactional
interface `reftable_stack_add()`:

  - It performs auto-compaction by itself.

  - It now transitively performs auto-compaction via the transactional
    interface.

Remove the first instance so that we only end up doing auto-compaction
once.

Reported-by: Han-Wen Nienhuys <hanwenn@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>