PERF: remove avg_time calculations and regular jobs from posts and topics

After careful analysis of large data-sets it became apparent that avg_time
had no impact whatsoever on "best of" topic scoring. Calculating avg_time
was a very costly operation especially on large databases.

We have some longer term plans of introducing other weighting that is read
time based into our scoring for "best of" and "top" topics, but in the
interim to stop a large amount of work that is not achieving any value we
are removing the jobs.

Column removal will follow once we decide on a new replacement metric.

lib/score_calculator.rb

@@ -6,7 +6,6 @@ class ScoreCalculator
       like_score: 15,
       incoming_link_count: 5,
       bookmark_count: 2,
-      avg_time: 0.05,
       reads: 0.2
     }
   end