reftable/generic: drop interface

reftable/generic: drop interface

The `reftable_table` interface provides a generic infrastructure that
can abstract away whether the underlying table is a single table, or a
merged table. This abstraction can make it rather hard to reason about
the code. We didn't ever use it to implement the reftable backend, and
with the preceding patches in this patch series we in fact don't use it
at all anymore. Furthermore, it became somewhat useless with the recent
refactorings that made it possible to seek reftable iterators multiple
times, as these now provide generic access to tables for us. The
interface is thus redundant and only brings unnecessary complexity with
it.

Remove the `struct reftable_table` interface and its associated
functions.

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