Tidy up SidekiqConfig::CliMethods#worker_queues

This doesn't need to support an array of strings any more; it was always
a very cautious approach.

lib/gitlab/sidekiq_config/cli_methods.rb

@@ -48,7 +48,6 @@ module Gitlab
       # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
       def worker_queues(rails_path = Rails.root.to_s)
-        # https://gitlab.com/gitlab-org/gitlab/issues/199230
         worker_names(all_queues(rails_path))
       end
 
@@ -75,7 +74,7 @@ module Gitlab
       private
 
       def worker_names(workers)
-        workers.map { |queue| queue.is_a?(Hash) ? queue[:name] : queue }
+        workers.map { |queue| queue[:name] }
       end
 
       def query_string_to_lambda(query_string)

spec/lib/gitlab/sidekiq_config/cli_methods_spec.rb

@@ -54,14 +54,6 @@ describe Gitlab::SidekiqConfig::CliMethods do
         end
       end
 
-      context 'when the file contains an array of strings' do
-        before do
-          stub_contents(['queue_a'], ['queue_b'])
-        end
-
-        include_examples 'valid file contents'
-      end
-
       context 'when the file contains an array of hashes' do
         before do
           stub_contents([{ name: 'queue_a' }], [{ name: 'queue_b' }])