Remove dead code from svn_diff_hunk_readline_diff_text().

Since the hunk header will never be included in the hunk->diff_text_range
there is no reason to try to deal with reversing it.

* subversion/libsvn_diff/parse-diff.c
  (svn_diff_hunk_readline_diff_text): Remove some always false conditional
    code and a variable it used.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1548689 13f79535-47bb-0310-9956-ffa450edef68