refs: move to using the '.optimize' functions

refs: move to using the '.optimize' functions

The `struct ref_store` variable exposes two ways to optimize a reftable
backend:

  1. pack_refs
  2. optimize

The former was specific to the 'files' + 'packed' refs backend. The
latter is more generic and covers all backends. While the naming is
different, both of these functions perform the same functionality.

Consolidate this code to only maintain the 'optimize' functions. Do this
by modifying the backends so that they exclusively implement the
`optimize` callback, only. All users of the refs subsystem already use
the 'optimize' function so there is no changes needed on the callee
side. Finally, cleanup all references to the 'pack_refs' field of the
structure and code around it.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>