refactor: slight clean up after GAing multiple roles (#5133)

Follow up cleanup to https://github.com/Unleash/unleash/pull/5109
Based on
https://github.com/Unleash/unleash/pull/5109#pullrequestreview-1689535863

frontend/cypress/integration/projects/access.spec.ts

@@ -66,19 +66,6 @@ describe('project-access', () => {
     beforeEach(() => {
         cy.login_UI();
 
-        cy.intercept('GET', `${baseUrl}/api/admin/ui-config`, (req) => {
-            req.headers['cache-control'] =
-                'no-cache, no-store, must-revalidate';
-            req.on('response', (res) => {
-                if (res.body) {
-                    res.body.flags = {
-                        ...res.body.flags,
-                        multipleRoles: true,
-                    };
-                }
-            });
-        });
-
         cy.visit(`/projects/${groupAndProjectName}/settings/access`);
         if (document.querySelector("[data-testid='CLOSE_SPLASH']")) {
             cy.get("[data-testid='CLOSE_SPLASH']").click();

frontend/src/component/project/ProjectAccess/ProjectAccessAssign/ProjectAccessAssign.tsx

@@ -36,7 +36,6 @@ import {
 import { caseInsensitiveSearch } from 'utils/search';
 import { IServiceAccount } from 'interfaces/service-account';
 import { MultipleRoleSelect } from 'component/common/MultipleRoleSelect/MultipleRoleSelect';
-import { RoleSelect } from 'component/common/RoleSelect/RoleSelect';
 
 const StyledForm = styled('form')(() => ({
     display: 'flex',

frontend/src/interfaces/uiConfig.ts

@@ -61,7 +61,6 @@ export type UiFlags = {
     customRootRolesKillSwitch?: boolean;
     strategyVariant?: boolean;
     lastSeenByEnvironment?: boolean;
-    multipleRoles?: boolean;
     featureNamingPattern?: boolean;
     doraMetrics?: boolean;
     variantTypeNumber?: boolean;