Remove unused AjaxLoadingSpinner

The last remaining instance of `.js-ajax-loading-spinner` was removed in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77231, so this
should be safe to delete entirely.

app/assets/javascripts/branches/ajax_loading_spinner.js

-import $ from 'jquery';
-
-export default class AjaxLoadingSpinner {
-  static init() {
-    const $elements = $('.js-ajax-loading-spinner');
-    $elements.on('ajax:beforeSend', AjaxLoadingSpinner.ajaxBeforeSend);
-  }
-
-  static ajaxBeforeSend(e) {
-    const button = e.target;
-    const newButton = document.createElement('button');
-    newButton.classList.add('btn', 'btn-default', 'disabled', 'gl-button');
-    newButton.setAttribute('disabled', 'disabled');
-
-    const spinner = document.createElement('span');
-    spinner.classList.add('align-text-bottom', 'gl-spinner', 'gl-spinner-sm', 'gl-spinner-orange');
-    newButton.appendChild(spinner);
-
-    button.classList.add('hidden');
-    button.parentNode.insertBefore(newButton, button.nextSibling);
-
-    $(button).one('ajax:error', () => {
-      newButton.remove();
-      button.classList.remove('hidden');
-    });
-
-    $(button).one('ajax:success', () => {
-      $(button).off('ajax:beforeSend', AjaxLoadingSpinner.ajaxBeforeSend);
-    });
-  }
-}

app/assets/javascripts/pages/projects/branches/index/index.js

 import initDeprecatedRemoveRowBehavior from '~/behaviors/deprecated_remove_row_behavior';
-import AjaxLoadingSpinner from '~/branches/ajax_loading_spinner';
 import BranchSortDropdown from '~/branches/branch_sort_dropdown';
 import initDiverganceGraph from '~/branches/divergence_graph';
 import initDeleteBranchButton from '~/branches/init_delete_branch_button';
 import initDeleteBranchModal from '~/branches/init_delete_branch_modal';
 
-AjaxLoadingSpinner.init();
-
 const { divergingCountsEndpoint, defaultBranch } = document.querySelector(
   '.js-branch-list',
 ).dataset;

spec/frontend/branches/ajax_loading_spinner_spec.js

-import AjaxLoadingSpinner from '~/branches/ajax_loading_spinner';
-
-describe('Ajax Loading Spinner', () => {
-  let ajaxLoadingSpinnerElement;
-  let fauxEvent;
-  beforeEach(() => {
-    document.body.innerHTML = `
-    <div>
-    <a class="js-ajax-loading-spinner"
-       data-remote
-       href="http://goesnowhere.nothing/whereami">
-       Remove me
-    </a></div>`;
-    AjaxLoadingSpinner.init();
-    ajaxLoadingSpinnerElement = document.querySelector('.js-ajax-loading-spinner');
-    fauxEvent = { target: ajaxLoadingSpinnerElement };
-  });
-
-  afterEach(() => {
-    document.body.innerHTML = '';
-  });
-
-  it('`ajaxBeforeSend` event handler sets current icon to spinner and disables link', () => {
-    expect(ajaxLoadingSpinnerElement.parentNode.querySelector('.gl-spinner')).toBeNull();
-    expect(ajaxLoadingSpinnerElement.classList.contains('hidden')).toBe(false);
-
-    AjaxLoadingSpinner.ajaxBeforeSend(fauxEvent);
-
-    expect(ajaxLoadingSpinnerElement.parentNode.querySelector('.gl-spinner')).not.toBeNull();
-    expect(ajaxLoadingSpinnerElement.classList.contains('hidden')).toBe(true);
-  });
-});