Remove `FF_LEGACY_PERSISTED_NAMES` feature flag

The new method of fetching feature flags
seems to behave correctly.

This is safe to remove the feature flag.

lib/feature.rb

@@ -34,18 +34,6 @@ class Feature
     def persisted_names
       return [] unless Gitlab::Database.exists?
 
-      if Gitlab::Utils.to_boolean(ENV['FF_LEGACY_PERSISTED_NAMES'])
-        # To be removed:
-        # This uses a legacy persisted names that are know to work (always)
-        Gitlab::SafeRequestStore[:flipper_persisted_names] ||=
-          begin
-            # We saw on GitLab.com, this database request was called 2300
-            # times/s. Let's cache it for a minute to avoid that load.
-            Gitlab::ProcessMemoryCache.cache_backend.fetch('flipper:persisted_names', expires_in: 1.minute) do
-              FlipperFeature.feature_names
-            end.to_set
-          end
-      else
       # This loads names of all stored feature flags
       # and returns a stable Set in the following order:
       # - Memoized: using Gitlab::SafeRequestStore or @flipper
@@ -54,7 +42,6 @@ class Feature
       # - DB: using a single SQL query
       flipper.adapter.features
     end
-    end
 
     def persisted_name?(feature_name)
       # Flipper creates on-memory features when asked for a not-yet-created one.

spec/lib/feature_spec.rb

@@ -21,11 +21,6 @@ RSpec.describe Feature, stub_feature_flags: false do
   end
 
   describe '.persisted_names' do
-    context 'when FF_LEGACY_PERSISTED_NAMES=false' do
-      before do
-        stub_env('FF_LEGACY_PERSISTED_NAMES', 'false')
-      end
-
     it 'returns the names of the persisted features' do
       Feature.enable('foo')
 
@@ -50,38 +45,6 @@ RSpec.describe Feature, stub_feature_flags: false do
         expect(described_class.persisted_names).to contain_exactly('foo')
       end
     end
-    end
-
-    context 'when FF_LEGACY_PERSISTED_NAMES=true' do
-      before do
-        stub_env('FF_LEGACY_PERSISTED_NAMES', 'true')
-      end
-
-      it 'returns the names of the persisted features' do
-        Feature.enable('foo')
-
-        expect(described_class.persisted_names).to contain_exactly('foo')
-      end
-
-      it 'returns an empty Array when no features are presisted' do
-        expect(described_class.persisted_names).to be_empty
-      end
-
-      it 'caches the feature names when request store is active',
-       :request_store, :use_clean_rails_memory_store_caching do
-        Feature.enable('foo')
-
-        expect(Gitlab::ProcessMemoryCache.cache_backend)
-          .to receive(:fetch)
-          .once
-          .with('flipper:persisted_names', expires_in: 1.minute)
-          .and_call_original
-
-        2.times do
-          expect(described_class.persisted_names).to contain_exactly('foo')
-        end
-      end
-    end
 
     it 'fetches all flags once in a single query', :request_store do
       Feature.enable('foo1')