Delete an outdated comment for read_property and read_dimension. [ci skip]

The comment is outdated as of PHP7.
For example, in the current code, zend_assign_op_overloaded_property() expects the refcount of the returned value from read_property to be greater than 0 when the returned value is a refcounted, to call zval_ptr_dtor() later.

History:
- The same description was originally written in a commit in 2004 alongside the write side. c8c0e97
- One requested to put the description in somewhere. https://externals.io/message/7789
- Then it was added as a comment in zend_object_handler.h . 7d3215d
- At the time the comment was written, there were at least three places where the code actually set the reference count to 0 in read_properties.
	- https://github.com/php/php-src/blob/7d3215d33321173c58db4d86b7398e16c5c55d13/ext/dom/php_dom.c#L229
	- https://github.com/php/php-src/blob/7d3215d33321173c58db4d86b7398e16c5c55d13/ext/mysqli/mysqli.c#L190
	- https://github.com/php/php-src/blob/7d3215d33321173c58db4d86b7398e16c5c55d13/ext/simplexml/simplexml.c#L245
- All three of the above were removed during the development of PHP 7.
	- 2f0a758
	- 2402d6c
	- a975c7e
- In the current code, even when they generate and return a refcounted value, its refcount would be 1.

Closes GH-6618.

Author: sj-i

Zend/zend_object_handlers.h

@@ -36,10 +36,6 @@ struct _zend_property_info;
 #define ZEND_ENCODE_DYN_PROP_OFFSET(offset)        ((uintptr_t)(-((intptr_t)(offset) + 2)))
 
 
-/* The following rule applies to read_property() and read_dimension() implementations:
-   If you return a zval which is not otherwise referenced by the extension or the engine's
-   symbol table, its reference count should be 0.
-*/
 /* Used to fetch property from the object, read-only */
 typedef zval *(*zend_object_read_property_t)(zend_object *object, zend_string *member, int type, void **cache_slot, zval *rv);