Remove some dead code

Usage of getters was removed in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/19827

ee/app/assets/javascripts/related_items_tree/store/getters.js

 import { issuableTypesMap, PathIdSeparator } from 'ee/related_issues/constants';
-import { ChildType } from '../constants';
 
 export const autoCompleteSources = () => gl.GfmAutoComplete && gl.GfmAutoComplete.dataSources;
 
@@ -8,21 +7,6 @@ export const directChildren = state => state.children[state.parentItem.reference
 export const anyParentHasChildren = (state, getters) =>
   getters.directChildren.some(item => item.hasChildren || item.hasIssues);
 
-export const headerItems = state => [
-  {
-    iconName: 'epic',
-    count: state.epicsCount,
-    qaClass: 'qa-add-epics-button',
-    type: ChildType.Epic,
-  },
-  {
-    iconName: 'issues',
-    count: state.issuesCount,
-    qaClass: 'qa-add-issues-button',
-    type: ChildType.Issue,
-  },
-];
-
 export const itemAutoCompleteSources = (state, getters) => {
   if (getters.isEpic) {
     return state.autoCompleteEpics ? getters.autoCompleteSources : {};

ee/spec/frontend/related_items_tree/store/getters_spec.js

 import * as getters from 'ee/related_items_tree/store/getters';
-
 import createDefaultState from 'ee/related_items_tree/store/state';
-
 import { issuableTypesMap } from 'ee/related_issues/constants';
-import { ChildType } from 'ee/related_items_tree/constants';
 
 import { mockEpic1, mockEpic2 } from '../../../javascripts/related_items_tree/mock_data';
 
@@ -60,36 +57,6 @@ describe('RelatedItemsTree', () => {
         });
       });
 
-      describe('headerItems', () => {
-        it('returns an item within array containing Epic iconName, count, qaClass & type props', () => {
-          state.epicsCount = 2;
-          const epicHeaderItem = getters.headerItems(state)[0];
-
-          expect(epicHeaderItem).toEqual(
-            expect.objectContaining({
-              iconName: 'epic',
-              count: 2,
-              qaClass: 'qa-add-epics-button',
-              type: ChildType.Epic,
-            }),
-          );
-        });
-
-        it('returns an item within array containing Issue iconName, count, qaClass & type props', () => {
-          state.issuesCount = 2;
-          const epicHeaderItem = getters.headerItems(state)[1];
-
-          expect(epicHeaderItem).toEqual(
-            expect.objectContaining({
-              iconName: 'issues',
-              count: 2,
-              qaClass: 'qa-add-issues-button',
-              type: ChildType.Issue,
-            }),
-          );
-        });
-      });
-
       describe('itemAutoCompleteSources', () => {
         it('returns autoCompleteSources value when `issuableType` is set to `Epic` and `autoCompleteEpics` is true', () => {
           const mockGetter = {