Revert "Reuse DEADAGE rather than defining our own constant."

This reverts commit b60f215d1fa2f04bee4c650efbba629a215fda6f.

This generates the following error:

 Traceback (most recent call last):
   File "/usr/lib/python2.6/wsgiref/handlers.py", line 94, in run
     self.finish_response()
   File "/usr/lib/python2.6/wsgiref/handlers.py", line 134, in finish_response
     for data in self.result:
   File "/home/build/master/web/../buildfarm/web/__init__.py", line 1178, in __call__
     yield "".join(self.html_page(form, page.render_html(myself)))
   File "/home/build/master/web/../buildfarm/web/__init__.py", line 1017, in html_page
     yield "".join(lines)
   File "/home/build/master/web/../buildfarm/web/__init__.py", line 757, in render_html
     (host_count, broken_count, panic_count) = self._get_counts()
   File "/home/build/master/web/../buildfarm/web/__init__.py", line 730, in _get_counts
     builds = self.buildfarm.get_summary_builds(min_age=time.time() - BuildFarm.DEADAGE)
 NameError: global name 'BuildFarm' is not defined

Signed-off-by: Stefan Metzmacher <metze@samba.org>