Remove SubmitEvent polyfill (#37276)

Remove this obsolete polyfill as per
https://github.com/go-gitea/gitea/pull/37270#issuecomment-4273399551.

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

web_src/js/index.ts

@@ -45,7 +45,6 @@ import {initCaptcha} from './features/captcha.ts';
 import {initRepositoryActionView} from './features/repo-actions.ts';
 import {initGlobalTooltips} from './modules/tippy.ts';
 import {initGiteaFomantic} from './modules/fomantic.ts';
-import {initSubmitEventPolyfill} from './utils/dom.ts';
 import {initRepoIssueList} from './features/repo-issue-list.ts';
 import {initCommonIssueListQuickGoto} from './features/common-issue-list.ts';
 import {initRepoContributors} from './features/contributors.ts';
@@ -69,7 +68,6 @@ import {initDevtest} from './modules/devtest.ts';
 
 const initStartTime = performance.now();
 const initPerformanceTracer = callInitFunctions([
-  initSubmitEventPolyfill,
   initGiteaFomantic,
 
   initGlobalComponent,