Delete issue board model classes

This is part of refactoring issue boards to Vuex (&1278 (closed)) and needs to happen after #28452 (closed).

All references to classes in app/assets/javascripts/boards/models/ need to be deleted and replaced by using the store directly (this requires updating components).

$ grep -r '/models/' {ee/,}app/assets/javascripts/boards
ee/app/assets/javascripts/boards/models/list.js:import List from '~/boards/models/list';
ee/app/assets/javascripts/boards/models/list.js:import ListAssignee from '~/vue_shared/models/assignee';
ee/app/assets/javascripts/boards/models/list.js:import ListMilestone from '~/boards/models/milestone';
ee/app/assets/javascripts/boards/models/issue.js:import ListIssue from '~/boards/models/issue';
app/assets/javascripts/boards/index.js:import '~/vue_shared/models/label';
app/assets/javascripts/boards/index.js:import '~/vue_shared/models/assignee';
app/assets/javascripts/boards/index.js:import './models/milestone';
app/assets/javascripts/boards/index.js:import './models/project';
app/assets/javascripts/boards/index.js:import 'ee/boards/models/list';
app/assets/javascripts/boards/index.js:import 'ee/boards/models/issue';
app/assets/javascripts/boards/index.js:import 'ee/boards/models/project';
app/assets/javascripts/boards/models/list.js:import ListLabel from '~/vue_shared/models/label';
app/assets/javascripts/boards/models/list.js:import ListAssignee from '~/vue_shared/models/assignee';
app/assets/javascripts/boards/models/issue.js:import '~/vue_shared/models/label';
app/assets/javascripts/boards/components/board_new_issue.vue:import ListIssue from '../models/issue';

$ grep -Er '/\* global .* \*/' {ee/,}app/assets/javascripts/boards
ee/app/assets/javascripts/boards/components/board_form.vue:/* global ListLabel */
app/assets/javascripts/boards/mixins/sortable_default_options.js:/* global DocumentTouch */
app/assets/javascripts/boards/stores/boards_store.js:/* global List */
app/assets/javascripts/boards/models/list.js:/* global ListIssue */
app/assets/javascripts/boards/models/issue.js:/* global ListLabel */
app/assets/javascripts/boards/models/issue.js:/* global ListMilestone */
app/assets/javascripts/boards/models/issue.js:/* global ListAssignee */
app/assets/javascripts/boards/components/board_blank_state.vue:/* global ListLabel */
app/assets/javascripts/boards/components/modal/index.vue:/* global ListIssue */