Remove store_merge_request_line_metrics FF

This change removes the store_merge_request_line_metrics feature flag.
The flag was default enabled since months. We forgot to clean it
up.

ee/config/feature_flags/development/store_merge_request_line_metrics.yml

----
-name: store_merge_request_line_metrics
-introduced_by_url: 
-rollout_issue_url: 
-group: 
-type: development
-default_enabled: true

ee/lib/analytics/merge_request_metrics_calculator.rb

@@ -19,8 +19,6 @@ module Analytics
 
     # rubocop: disable CodeReuse/ActiveRecord
     def line_counts_data
-      return {} if Feature.disabled?(:store_merge_request_line_metrics, merge_request.target_project, default_enabled: true)
-
       {
         added_lines: raw_diff_files.sum(&:added_lines),
         removed_lines: raw_diff_files.sum(&:removed_lines)

ee/spec/services/ee/merge_request_metrics_service_spec.rb

@@ -33,19 +33,6 @@ RSpec.describe EE::MergeRequestMetricsService do
       it 'updates `removed_lines`' do
         expect { subject }.to change { merge_request.metrics.removed_lines }.from(nil).to(expected_removed_lines)
       end
-
-      context 'when `store_merge_request_line_metrics` feature flag is disabled' do
-        before do
-          stub_feature_flags(store_merge_request_line_metrics: false)
-        end
-
-        it 'does not update line counts' do
-          subject
-
-          expect(merge_request.metrics.added_lines).to be_nil
-          expect(merge_request.metrics.removed_lines).to be_nil
-        end
-      end
     end
   end
 end