Remove unused parameter.

* subversion/libsvn_fs_fs/index.c
  (read_entry): POOL parameter is not used -> remove it.
  (get_p2l_page): Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1626873 13f79535-47bb-0310-9956-ffa450edef68