Remove find_unused_test_files.py. NFC (#26385)

Somewhat surprisingly I ended up writing the same script twice here.

This change deletes one of them and updates the other little.

Author: sbc100

tools/maint/check_for_unused_test_files.py

@@ -27,8 +27,14 @@
 ignore_root_patterns = ['runner.*', 'test_*.py']
 ignore_root_files = {
   'check_clean.py',
+  'decorators.py',
   'jsrun.py',
   'clang_native.py',
+  'line_endings.py',
+  'single_line_runner.py',
+  'color_runner.py',
+  'retryable_unittest.py',
+  'browser_common.py',
   'common.py',
   'parallel_testsuite.py',
   'parse_benchmark_output.py',