Remove svn_repos__repos_pool_t private API and implementation.
Should we need something similar in the future, it's just a few LOC to add.

* subversion/include/private/svn_repos_private.h
  (svn_repos__repos_pool_t,
   svn_repos__repos_pool_create,
   svn_repos__repos_pool_get): Remove API.

* subversion/libsvn_repos/repos_pool.c
  (): Remove this file.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1581940 13f79535-47bb-0310-9956-ffa450edef68