Remove the request_deadline feature flag

The feature has been enabled on GitLab.com for over 24h without
issue.

It hasn't had the impact we hoped it would, but it does improve the
situation. And the calculated deadline can be reused in other parts of
the application that should enforce this limit.

changelogs/unreleased/bvl-remove-request-deadline-ff.yml

+---
+title: Don't allow Gitaly calls to exceed the worker timeout set for unicorn or puma
+merge_request: 23510
+author:
+type: performance

lib/gitlab/request_context.rb

@@ -18,7 +18,6 @@ module Gitlab
     def request_deadline
       strong_memoize(:request_deadline) do
         next unless request_start_time
-        next unless Feature.enabled?(:request_deadline)
 
         request_start_time + max_request_duration_seconds
       end

spec/lib/gitlab/request_context_spec.rb

@@ -24,18 +24,6 @@ describe Gitlab::RequestContext, :request_store do
 
       expect(subject.request_deadline).to be_nil
     end
-
-    it 'only checks the feature once per request-instance' do
-      expect(Feature).to receive(:enabled?).with(:request_deadline).once
-
-      2.times { subject.request_deadline }
-    end
-
-    it 'returns nil when the feature is disabled' do
-      stub_feature_flags(request_deadline: false)
-
-      expect(subject.request_deadline).to be_nil
-    end
   end
 
   describe '#ensure_request_deadline_not_exceeded!' do