reftable/reader: inline `reader_close()`

reftable/reader: inline `reader_close()`

Same as with the preceding commit, we also provide a `reader_close()`
function that allows the caller to close a reader without freeing it.
This is unnecessary now that all users will have an allocated version of
the reader.

Inline it into `reftable_reader_free()`.

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