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>

tests/integration/migration-test/migration_test.go

@@ -36,8 +36,6 @@ var currentEngine *xorm.Engine
 
 func initMigrationTest(t *testing.T) func() {
 	testlogger.Init()
-	setting.SetupGiteaTestEnv()
-
 	unittest.InitSettingsForTesting()
 
 	assert.NotEmpty(t, setting.RepoRootPath)