Merge a no-longer-useful error construction helper function into its only
remaining caller.

* subversion/libsvn_repos/delta.c
  (not_a_dir_error): Delete.
  (svn_repos_dir_delta2): Write the equivalent error return in-line here.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1480780 13f79535-47bb-0310-9956-ffa450edef68