remove last_activity from CountsReport

This data point is difficult to get with the new page views
infrastructure, and isn't displayed anywhere anyway.

fixes CNVS-6731

test plan:
* with cassandra page views enabled, open a rails console and run
  Reporting::CountsReport.process
* once that completes, go to /accounts/X/statistics and verify that the
  stats have been updated (there is a generated timestamp at the top)

Change-Id: I7f1144cd6dab9b68478a79e3f7f82bc6d8feb0b3
Reviewed-on: https://gerrit.instructure.com/22046
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>

lib/reporting/counts_report.rb

@@ -46,15 +46,12 @@ class CountsReport
       Shard.with_each_shard do
         Account.root_accounts.active.each do |account|
           next if account.external_status == 'test'
-          activity = CountsReport.last_activity(account.id)
-          next unless activity
 
           data = {}.with_indifferent_access
           data[:generated_at] = @timestamp
           data[:id] = account.id
           data[:name] = account.name
           data[:external_status] = account.external_status
-          data[:last_activity] = activity
 
           course_ids = get_course_ids(account)
           data[:courses] = course_ids.length
@@ -212,10 +209,6 @@ class CountsReport
     @overview[:totals][:users] += account[:users]
   end
 
-  def self.last_activity(account_id)
-    PageView.where(:account_id => account_id).maximum(:created_at)
-  end
-
   def get_course_ids(account)
     is_default_account = account.external_status == ExternalStatuses.default_external_status.to_s
     course_ids = []