Cleanup some unnecessary functions and calls due to changes to the stream API.

* subversion/libsvn_wc/translate.c
  (read_handler_unsupported, write_handler_unsupported): Remove.
  (svn_wc__internal_translated_stream): Don't set bogus read/write handlers to
    block unsupported uses of the streams, since the stream API now returns an
    error now.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568571 13f79535-47bb-0310-9956-ffa450edef68