reftable/blocksource: drop malloc block source

reftable/blocksource: drop malloc block source

The reftable blocksource provides a generic interface to read blocks via
different sources, e.g. from disk or from memory. One of the block
sources is the malloc block source, which can in theory read data from
memory. We nowadays also have a strbuf block source though, which
provides essentially the same functionality with better ergonomics.

Adapt the only remaining user of the malloc block source in our tests
to use the strbuf block source, instead, and remove the now-unused
malloc block source.

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