Fix message in test

Looks like I did not "git add" the message update.

Also drop the now dead ZEND_ENUM_PROPERTY_ERROR() macro while
here.

Zend/tests/enum/no-write-properties-cache-slot.phpt

@@ -23,5 +23,5 @@ var_dump(Test::A->name);
 ?>
 --EXPECT--
 string(1) "A"
-Enum properties are immutable
+Cannot modify readonly property Test::$name
 string(1) "A"

Zend/zend_enum.c

@@ -22,9 +22,6 @@
 #include "zend_enum_arginfo.h"
 #include "zend_interfaces.h"
 
-#define ZEND_ENUM_PROPERTY_ERROR() \
-	zend_throw_error(NULL, "Enum properties are immutable")
-
 #define ZEND_ENUM_DISALLOW_MAGIC_METHOD(propertyName, methodName) \
 	do { \
 		if (ce->propertyName) { \