ext/standard: drop old smart_string headers

Functionality was removed in PHP 7.2

Author: DanielEScherzer

UPGRADING.INTERNALS

@@ -72,6 +72,10 @@ PHP 8.5 INTERNALS UPGRADE NOTES
     * zend_register_stringl_constant()
   . EG(fake_scope) now is a _const_ zend_class_entry*.
 
+- standard
+  . ext/standard/php_smart_string.h and ext/standard/php_smart_string_public.h
+    were removed. Use the corresponding headers in Zend/ instead.
+
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