Use exported DEFAULT_PER_PAGE constant

Switch from default export to named export and delete
redundant file where this constant was define with the same value.

app/assets/javascripts/api/constants.js

-export const DEFAULT_PER_PAGE = 20;

app/assets/javascripts/api/groups_api.js

+import { DEFAULT_PER_PAGE } from '~/api';
 import axios from '../lib/utils/axios_utils';
 import { buildApiUrl } from './api_utils';
-import { DEFAULT_PER_PAGE } from './constants';
 
 const GROUPS_PATH = '/api/:version/groups.json';
 const DESCENDANT_GROUPS_PATH = '/api/:version/groups/:id/descendant_groups';

app/assets/javascripts/api/projects_api.js

+import { DEFAULT_PER_PAGE } from '~/api';
 import axios from '../lib/utils/axios_utils';
 import { buildApiUrl } from './api_utils';
-import { DEFAULT_PER_PAGE } from './constants';
 
 const PROJECTS_PATH = '/api/:version/projects.json';
 

app/assets/javascripts/api/user_api.js

+import { DEFAULT_PER_PAGE } from '~/api';
 import createFlash from '~/flash';
 import { __ } from '~/locale';
 import axios from '../lib/utils/axios_utils';
 import { buildApiUrl } from './api_utils';
-import { DEFAULT_PER_PAGE } from './constants';
 
 const USER_COUNTS_PATH = '/api/:version/user_counts';
 const USERS_PATH = '/api/:version/users.json';

ee/app/assets/javascripts/api.js

-import Api from '~/api';
+import Api, { DEFAULT_PER_PAGE } from '~/api';
 import axios from '~/lib/utils/axios_utils';
 import { ContentTypeMultipartFormData } from '~/lib/utils/headers';
 
@@ -61,7 +61,7 @@ export default {
       .get(url, {
         params: {
           search: query,
-          per_page: Api.DEFAULT_PER_PAGE,
+          per_page: DEFAULT_PER_PAGE,
           active: true,
         },
       })
@@ -379,7 +379,7 @@ export default {
   fetchBillableGroupMembersList(namespaceId, options = {}, callback = () => {}) {
     const url = Api.buildUrl(this.billableGroupMembersPath).replace(':id', namespaceId);
     const defaults = {
-      per_page: Api.DEFAULT_PER_PAGE,
+      per_page: DEFAULT_PER_PAGE,
       page: 1,
     };