Disable installing docs with distutils.

Without this patch, the package would install README and COPYING to /usr.
We're already installing the README file to /usr/share/doc/trac-mercurial with
dh_installdocs.

diff -r 4dfaed967cef setup.py
--- a/setup.py	Sun Jul 20 14:35:35 2008 -0600
+++ b/setup.py	Sun Jul 20 14:37:50 2008 -0600
@@ -21,5 +21,4 @@
       """ % TracMercurial,
       namespace_packages=['tracext'],
       packages=['tracext', 'tracext.hg'],
-      data_files=['COPYING', 'README'],
       entry_points={'trac.plugins': 'hg = tracext.hg.backend'})
