Remove support for custom assert() function

Deprecated in PHP 7.3 as part of
https://wiki.php.net/rfc/deprecations_php_7_3.

Author: nikic

Zend/tests/custom_assert_deprecation.phpt renamed to Zend/tests/custom_assert_forbidden.phpt

@@ -9,4 +9,4 @@ function assert() {}
 
 ?>
 --EXPECTF--
-Deprecated: Defining a custom assert() function is deprecated, as the function has special semantics in %s on line %d
+Fatal error: Defining a custom assert() function is not allowed, as the function has special semantics in %s on line %d

Zend/zend_compile.c

@@ -5740,8 +5740,8 @@ static void zend_begin_func_decl(znode *result, zend_op_array *op_array, zend_as
 	}
 
 	if (zend_string_equals_literal_ci(unqualified_name, "assert")) {
-		zend_error(E_DEPRECATED,
-			"Defining a custom assert() function is deprecated, "
+		zend_error(E_COMPILE_ERROR,
+			"Defining a custom assert() function is not allowed, "
 			"as the function has special semantics");
 	}