Remove catfile feature flag

533c1948 introduced a new feature flag
on this repository for the catfile cache. However, this flag has been
removed from the Gitaly code base a while ago and is thus void.

This change removes a lingering test referencing this test, as well as
the flags YAML defintion.

config/feature_flags/development/gitaly_catfile-cache.yml

----
-name: gitaly_catfile-cache
-introduced_by_url: 
-rollout_issue_url: 
-milestone: 
-type: development
-group: 
-default_enabled: false

spec/lib/gitlab/gitaly_client_spec.rb

@@ -267,11 +267,6 @@ RSpec.describe Gitlab::GitalyClient do
   end
 
   describe '.request_kwargs' do
-    context 'when catfile-cache feature is enabled' do
-      before do
-        stub_feature_flags('gitaly_catfile-cache': true)
-      end
-
     it 'sets the gitaly-session-id in the metadata' do
       results = described_class.request_kwargs('default', timeout: 1)
       expect(results[:metadata]).to include('gitaly-session-id')
@@ -294,7 +289,6 @@ RSpec.describe Gitlab::GitalyClient do
         end
       end
     end
-    end
 
     context 'gitlab_git_env' do
       let(:policy) { 'gitaly-route-repository-accessor-policy' }