Remove the now unused svn__strtol.

* subversion/include/private/svn_string_private.h
  (svn__strtol): Remove private API.

* subversion/include/svn_types.h
  (SVN_STR_TO_REV): Don't mention the removed function in the docstring.

* subversion/libsvn_subr/string.c
  (svn__strtol): Remove implementation.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618856 13f79535-47bb-0310-9956-ffa450edef68