Hide some essentially dead code by using the preprocessor.

* subversion/libsvn_subr/cmdline.c
  (CMDLINE_USE_CUSTOM_ENCODING): New macro. Document why this still exists.
  (input_encoding,
   output_encoding): Disable when CMDLINE_USE_CUSTOM_ENCODING is not defined.

  (svn_cmdline_init): Use CMDLINE_USE_CUSTOM_ENCODING as condition around
    initializing the encoding usage.

  (svn_cmdline_cstring_from_utf8,
   svn_cmdline_cstring_to_utf8,
   svn_cmdline_output_encoding): Use preprocessor instead of runtime check
    for the 99.9% case.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1530582 13f79535-47bb-0310-9956-ffa450edef68