Remove unused SidekiqLogger

Grep'd the project and don't see any calls for SidekiqLogger.

Removing it since it can be confused with `Sidekiq.logger`.

lib/gitlab/sidekiq_logger.rb

-# frozen_string_literal: true
-
-module Gitlab
-  class SidekiqLogger < Gitlab::Logger
-    def self.file_name_noext
-      'sidekiq'
-    end
-  end
-end