reftable: inline `merged_table_release()`

reftable: inline `merged_table_release()`

The function `merged_table_release()` releases a merged table, whereas
`reftable_merged_table_free()` releases a merged table and then also
free's its pointer. But all callsites of `merged_table_release()` are in
fact followed by `reftable_merged_table_free()`, which is redundant.

Inline `merged_table_release()` into `reftable_merged_table_free()` to
get rid of this redundance.

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