Deprecate 3 unused Django settings

This commit deprecates 3 unused Django settings:

OFFLINE_OUTPUT_DIR = ''
GOOGLE_ANALYTICS_ID = ''
GOOGLE_ANALYTICS_SITE = ''

These were all added back in April 2016 (!) but as far as I can tell
are not used anywhere. Therefore we can remove them.

cfgov/cfgov/settings/base.py

@@ -460,15 +460,8 @@
 # override this based on the SAML_AUTH environment variable.
 SAML_AUTH = False
 
-# When we generate an full HTML version of the regulation, we want to
-# write it out somewhere. This is where.
-OFFLINE_OUTPUT_DIR = ""
-
 DATE_FORMAT = "n/j/Y"
 
-GOOGLE_ANALYTICS_ID = ""
-GOOGLE_ANALYTICS_SITE = ""
-
 # CDNs
 WAGTAILFRONTENDCACHE = {}