remove the rake task `gitlab:sidekiq:drop_post_receive`

This task is no longer being used and is not documented.

lib/tasks/gitlab/sidekiq.rake

-namespace :gitlab do
-  namespace :sidekiq do
-    QUEUE = 'queue:post_receive'.freeze
-
-    desc 'Drop all Sidekiq PostReceive jobs for a given project'
-    task :drop_post_receive, [:project] => :environment do |t, args|
-      unless args.project.present?
-        abort "Please specify the project you want to drop PostReceive jobs for:\n  rake gitlab:sidekiq:drop_post_receive[group/project]"
-      end
-      project_path = Project.find_by_full_path(args.project).repository.path_to_repo
-
-      Sidekiq.redis do |redis|
-        unless redis.exists(QUEUE)
-          abort "Queue #{QUEUE} is empty"
-        end
-
-        temp_queue = "#{QUEUE}_#{Time.now.to_i}"
-        redis.rename(QUEUE, temp_queue)
-
-        # At this point, then post_receive queue is empty. It may be receiving
-        # new jobs already. We will repopulate it with the old jobs, skipping the
-        # ones we want to drop.
-        dropped = 0
-        while (job = redis.lpop(temp_queue))
-          if repo_path(job) == project_path
-            dropped += 1
-          else
-            redis.rpush(QUEUE, job)
-          end
-        end
-        # The temp_queue will delete itself after we have popped all elements
-        # from it
-
-        puts "Dropped #{dropped} jobs containing #{project_path} from #{QUEUE}"
-      end
-    end
-
-    def repo_path(job)
-      job_args = JSON.parse(job)['args']
-      if job_args
-        job_args.first
-      else
-        nil
-      end
-    end
-  end
-end