Fix "unused function" compiler warnings.

* subversion/libsvn_ra_svn/marshal.c
  (write_tuple_word,
   write_tuple_word_opt,
   write_tuple_number): remove unused functions

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1465134 13f79535-47bb-0310-9956-ffa450edef68