Drop the cache-instance private pool we used for tempoary allocations
and would clean periodically.

It is simply no longer needed as we get an external scratch pool passed
in and the temp. allocations may actually be significant if the object
to serialize is large and complex.

* subversion/libsvn_subr/cache-membuffer.c
  (svn_membuffer_cache_t): Remove private pool and associated usage counter.
  (svn_membuffer_cache_set): Simply use the scratch_pool during serialization
                             and remove all private pool code.
  (svn_cache__create_membuffer_cache): Drop obsolete init code.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1564217 13f79535-47bb-0310-9956-ffa450edef68