Clean up Makefile, tests and legacy code (#36638)

This simplifies the Makefile by removing the whole-file wrapping that
creates a tempdir introduced by
https://github.com/go-gitea/gitea/pull/11126. REPO_TEST_DIR is removed
as well.

Also clean up a lot of legacy code: unnecessary XSS test, incorrect test
env init, unused "_old_uid" hack, etc

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

tsconfig.json

@@ -44,6 +44,7 @@
     "stripInternal": true,
     "verbatimModuleSyntax": true,
     "types": [
+      "node",
       "webpack/module",
       "vitest/globals",
       "./web_src/js/globals.d.ts",