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>

pnpm-lock.yaml

@@ -234,6 +234,9 @@ importers:
       '@types/katex':
         specifier: 0.16.8
         version: 0.16.8
+      '@types/node':
+        specifier: 25.2.3
+        version: 25.2.3
       '@types/pdfobject':
         specifier: 2.2.5
         version: 2.2.5