AI: Remove dead polyfills.php from php-ai-client.

Trac ticket: https://core.trac.wordpress.org/ticket/64591

The file is never loaded because the generated autoloader only handles
class autoloading, and the four polyfilled functions (`array_is_list`,
`str_starts_with`, `str_contains`, `str_ends_with`) are already provided
by WordPress Core in `wp-includes/compat.php`.

Also updates `installer.sh` to strip the file during future library updates.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: gziolo

src/wp-includes/php-ai-client/src/polyfills.php

@@ -225,6 +225,9 @@ for path in "${REMOVE_PATHS[@]}"; do
 	rm -rf "$TARGET_DIR/third-party/$path"
 done
 
+# Remove polyfills.php — WordPress Core already provides these functions in compat.php.
+rm -f "$TARGET_DIR/src/polyfills.php"
+
 # ---------------------------------------------------------------------------
 # Generate autoload.php
 # ---------------------------------------------------------------------------