Remove duplicated information from a few error messages.

* subversion/libsvn_subr/io.c
  (svn_io_wait_for_cmd): Remove 'exitwhy' from the error message for the cases
    where exitwhy is already completely handled by the specific error message.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1489935 13f79535-47bb-0310-9956-ffa450edef68