Remove php_pdo_mysql_sqlstate.h

This file is unused (at least as of PHP 8.0.0); we also remove the
script that could generate it, and the respective info from
CONTRIBUTING.md.

Closes GH-10363.

Author: marcosmarcolin

CONTRIBUTING.md

@@ -185,9 +185,6 @@ locations.
     └─ pcre/
        ├─ pcre2lib/                 # https://www.pcre.org/
        └─ ...
-    └─ pdo_mysql/
-       ├─ php_pdo_mysql_sqlstate.h  # Generated by `ext/pdo_mysql/get_error_codes.php`
-       └─ ...
     └─ skeleton/                    # Skeleton for developing new extensions with `ext/ext_skel.php`
        └─ ...
     └─ standard/