release_delta_base_cache: reuse existing detach function

release_delta_base_cache: reuse existing detach function

This function drops an entry entirely from the cache,
meaning that aside from the freeing of the buffer, it is
exactly equivalent to detach_delta_base_cache_entry(). Let's
build on top of the detach function, which shortens the code
and will make it simpler when we change out the underlying
storage in future patches.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>