Removing test for ref diff

This test was testing a feature flag and is now not needed.
The feature flag is removed and the feature is default behaviour.
It is tested on lower leves.

qa/qa/specs/features/browser_ui/3_create/merge_request/view_merge_request_merge_ref_diff_spec.rb

-# frozen_string_literal: true
-
-module QA
-  RSpec.describe 'Create', :requires_admin, quarantine: { issue: 'https://gitlab.com/gitlab-org/gitlab/-/issues/261793', type: :investigating } do
-    describe 'View merge request merge-ref diff' do
-      let(:project) do
-        Resource::Project.fabricate_via_api! do |project|
-          project.name = 'merge-ref-diff'
-        end
-      end
-
-      let(:merge_request) do
-        Resource::MergeRequest.fabricate_via_api! do |merge_request|
-          merge_request.project = project
-          merge_request.title = 'This is a merge request'
-          merge_request.description = '... for viewing merge-ref and merge-base diffs'
-          merge_request.file_content = 'This exists on the source branch only'
-        end
-      end
-
-      let(:new_file_name) { "added_file-#{SecureRandom.hex(8)}.txt" }
-
-      context 'when the feature flag default_merge_ref_for_diffs is enabled' do
-        before do
-          Runtime::Feature.enable('default_merge_ref_for_diffs', project: project)
-
-          commit_to_branch(merge_request.target_branch, new_file_name)
-          commit_to_branch(merge_request.source_branch, new_file_name)
-
-          Flow::Login.sign_in
-
-          merge_request.visit!
-        end
-
-        it 'views the merge-ref diff by default', testcase: 'https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/347651' do
-          Page::MergeRequest::Show.perform do |mr_page|
-            mr_page.click_diffs_tab
-            mr_page.click_target_version_dropdown
-
-            expect(mr_page.version_dropdown_content).to include("#{project.default_branch} (HEAD)")
-            expect(mr_page.version_dropdown_content).not_to include("#{project.default_branch} (base)")
-            expect(mr_page).to have_file(merge_request.file_name)
-            expect(mr_page).not_to have_file(new_file_name)
-          end
-        end
-      end
-
-      context 'when the feature flag default_merge_ref_for_diffs is disabled' do
-        before do
-          Runtime::Feature.disable('default_merge_ref_for_diffs', project: project)
-
-          commit_to_branch(merge_request.target_branch, new_file_name)
-          commit_to_branch(merge_request.source_branch, new_file_name)
-
-          Flow::Login.sign_in
-
-          merge_request.visit!
-        end
-
-        it 'views the merge-base diff by default', testcase: 'https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/347650' do
-          Page::MergeRequest::Show.perform do |mr_page|
-            mr_page.click_diffs_tab
-            mr_page.click_target_version_dropdown
-
-            expect(mr_page.version_dropdown_content).to include("#{project.default_branch} (HEAD)")
-            expect(mr_page.version_dropdown_content).to include("#{project.default_branch} (base)")
-            expect(mr_page).to have_file(merge_request.file_name)
-            expect(mr_page).to have_file(new_file_name)
-          end
-        end
-      end
-
-      def commit_to_branch(branch, file)
-        Resource::Repository::Commit.fabricate_via_api! do |commit|
-          commit.project = merge_request.project
-          commit.branch = branch
-          commit.commit_message = "Add new file on #{branch}"
-          commit.add_files(
-            [
-                {
-                    file_path: file,
-                    content: "This exists on source and target branches"
-                }
-            ]
-          )
-        end
-      end
-    end
-  end
-end