Remove the currently unused svn_file_t code from /trunk.

* subversion/include/private/svn_file.h,
  subversion/libsvn_subr/file.c: Drop.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618578 13f79535-47bb-0310-9956-ffa450edef68