Remove the service worker (#25010)

It's been disabled by default since 1.17
(https://github.com/go-gitea/gitea/pull/18914), and it never really
delivered any benefit except being another cache layer that has its own
unsolved invalidation issues. HTTP cache works, we don't need two cache
layers at the browser for assets.

## ⚠️ BREAKING

You can remove the config `[ui].USE_SERVICE_WORKER` from your `app.ini`
now.

web_src/js/index.js

@@ -11,7 +11,6 @@ import {initHeatmap} from './features/heatmap.js';
 import {initImageDiff} from './features/imagediff.js';
 import {initRepoMigration} from './features/repo-migration.js';
 import {initRepoProject} from './features/repo-projects.js';
-import {initServiceWorker} from './features/serviceworker.js';
 import {initTableSort} from './features/tablesort.js';
 import {initAdminUserListSearchForm} from './features/admin/users.js';
 import {initAdminConfigs} from './features/admin/config.js';
@@ -116,7 +115,6 @@ onDomReady(() => {
   initImageDiff();
   initMarkupAnchors();
   initMarkupContent();
-  initServiceWorker();
   initSshKeyFormParser();
   initStopwatch();
   initTableSort();