t/helper: inline `reftable_stack_print_directory()`

t/helper: inline `reftable_stack_print_directory()`

Move `reftable_stack_print_directory()` into the "dump-reftable" helper.
This follows the same reasoning as the preceding commit.

Note that this requires us to remove the tests for this functionality in
`reftable/stack_test.c`. The test does not really add much anyway,
because all it verifies is that we do not crash or run into an error,
and it specifically doesn't check the outputted data. Also, as the code
is now part of the test helper, it doesn't make much sense to have a
unit test for it in the first place.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>