Delete the old zulip.com "referrals" system.

This system hasn't been in active use for several years, and had some 
problems with it's design.  So it makes sense to just remove it to declutter
the codebase.

Fixes #5655.

tools/js-dep-visualizer.py

@@ -127,7 +127,6 @@ def find_edges_to_remove(graph, methods):
         ('pm_list', 'resize'),
         ('notifications', 'navigate'),
         ('compose', 'socket'),
-        ('referral', 'resize'),
         ('stream_muting', 'message_util'),
         ('subs', 'stream_list'),
         ('ui', 'message_fetch'),