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.

Index: trac-mercurial-0.12.0.28/setup.py
===================================================================
--- trac-mercurial-0.12.0.28.orig/setup.py	2011-02-28 21:47:33.000000000 +0100
+++ trac-mercurial-0.12.0.28/setup.py	2011-12-14 16:14:49.057463238 +0100
@@ -41,7 +41,6 @@
       namespace_packages=['tracext'],
       packages=['tracext', 'tracext.hg'],
       package_data={
-          '': ['COPYING', 'README'],
           'tracext.hg': ['locale/*.*', 'locale/*/LC_MESSAGES/*.*'],
           },
       entry_points={'trac.plugins': 'hg = tracext.hg.backend'},
