Remove skip_legacy_diff_note_callback_on_import feature flag

- no longer needed after investigaton

Changelog: changed

app/models/legacy_diff_note.rb

@@ -91,7 +91,7 @@ class LegacyDiffNote < Note
   end
 
   def skip_setting_st_diff?
-    st_diff.present? && importing? && Feature.enabled?(:skip_legacy_diff_note_callback_on_import, default_enabled: :yaml)
+    st_diff.present? && importing?
   end
 
   def diff_for_line_code

config/feature_flags/development/skip_legacy_diff_note_callback_on_import.yml

----
-name: skip_legacy_diff_note_callback_on_import
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/72897
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/343666
-milestone: '14.5'
-type: development
-group: group::import
-default_enabled: false

spec/models/legacy_diff_note_spec.rb

@@ -36,18 +36,6 @@ RSpec.describe LegacyDiffNote do
           expect(note.st_diff).to eq('_st_diff_')
         end
 
-        context 'when feature flag is false' do
-          before do
-            stub_feature_flags(skip_legacy_diff_note_callback_on_import: false)
-          end
-
-          it 'updates st_diff' do
-            note.save!(validate: false)
-
-            expect(note.st_diff).to eq({})
-          end
-        end
-
         context 'when st_diff is blank' do
           before do
             note.st_diff = nil