Revert breaking prometheus changes to gunicorn_conf.py

[dannyroberts]

SUMMARY

Reverts this aspect of: #26876

FEATURE FLAG

PRODUCT DESCRIPTION

[dannyroberts]

@snopoke I expect you'll want to add this back, the offending failure was

Traceback (most recent call last):
  File "/home/cchq/www/staging/releases/2020-03-19_20.33/python_env-3.6/bin/gunicorn", line 10, in <module>
    sys.exit(run())
  File "/home/cchq/www/staging/releases/2020-03-19_20.33/python_env-3.6/lib/python3.6/site-packages/gunicorn/app/wsgiapp.py", line 58, in run
    WSGIApplication("%(prog)s [OPTIONS] [APP_MODULE]").run()
  File "/home/cchq/www/staging/releases/2020-03-19_20.33/python_env-3.6/lib/python3.6/site-packages/gunicorn/app/base.py", line 219, in run
    super().run()
  File "/home/cchq/www/staging/releases/2020-03-19_20.33/python_env-3.6/lib/python3.6/site-packages/gunicorn/app/base.py", line 72, in run
    Arbiter(self).run()
  File "/home/cchq/www/staging/releases/2020-03-19_20.33/python_env-3.6/lib/python3.6/site-packages/gunicorn/arbiter.py", line 198, in run
    self.start()
  File "/home/cchq/www/staging/releases/2020-03-19_20.33/python_env-3.6/lib/python3.6/site-packages/gunicorn/arbiter.py", line 138, in start
    self.cfg.on_starting(self)
  File "deployment/gunicorn/gunicorn_conf.py", line 29, in on_starting
    for f in glob.glob(os.path.join(path, '*.db')):
  File "/home/cchq/www/staging/releases/2020-03-19_20.33/python_env-3.6/lib/python3.6/posixpath.py", line 80, in join
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not NoneType

on gunicorn start up (but not runserver) which I traced back to this. I think as a precaution, any fix you add here you should make sure to deploy to staging just to make sure, sound good?

[snopoke]

Thanks @dannyroberts. I had tested this locally but must have not cleared the env var properly