Remove pushes count from usage ping payload

This query is constantly generating timeout errors on large installations and we don't
have a simple solution for now and also we don't think having this counter is
really critical.

lib/gitlab/usage_data.rb

@@ -32,7 +32,6 @@ module Gitlab
             merge_requests: MergeRequest.count,
             milestones: Milestone.count,
             notes: Note.count,
-            pushes: Event.code_push.count,
             pages_domains: PagesDomain.count,
             projects: Project.count,
             protected_branches: ProtectedBranch.count,

spec/lib/gitlab/usage_data_spec.rb

@@ -41,7 +41,6 @@ describe Gitlab::UsageData do
         milestones
         notes
         projects
-        pushes
         pages_domains
         protected_branches
         releases