Remove massive SscanfFormatValidationTest

The 1153-line test with a C ValidateFormat port, multiple algorithm
cross-validators, and 60+ format string database entries is excessive
for the scope of this fix. The actual behavioral coverage is already
provided by:

- tests/PHPStan/Analyser/nsrt/bug-14567.php (type inference assertions)
- tests/PHPStan/Analyser/nsrt/sscanf.php (specifier type assertions)
- tests/PHPStan/Rules/Functions/data/bug-14567.php (parameter count rule)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot