Simplify php_v8js_weak_object_callback.

This also fixes a leak in the `object_dom` testcase, where the DOM extension
returns an object with refcnt 1 but object_store refcnt 2, causing
READY_TO_DESTROY to return false.  Don't try to be fancy, just use the
standard PHP destructor.
Merge pull request #74 from cscott/dtor-simple

Simplify php_v8js_weak_object_callback.