remove php slim generator (deprecated) (#16108)

Author: wing328

docs/generators.md

@@ -119,7 +119,6 @@ The following generators are available:
 * [php-laravel](generators/php-laravel.md)  
 * [php-lumen](generators/php-lumen.md)  
 * [php-mezzio-ph](generators/php-mezzio-ph.md)  
-* [php-slim-deprecated (deprecated)](generators/php-slim-deprecated.md)  
 * [php-slim4](generators/php-slim4.md)  
 * [php-symfony](generators/php-symfony.md)  
 * [python-aiohttp](generators/python-aiohttp.md)