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

In 5c08645 (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 <[email protected]>
Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

reftable/stack.c

@@ -425,9 +425,6 @@ int reftable_stack_add(struct reftable_stack *st,
 		return err;
 	}
 
-	if (!st->disable_auto_compact)
-		return reftable_stack_auto_compact(st);
-
 	return 0;
 }