Add wxWeakRef::element_type, remove unnecessary assignment operator.

Add wxWeakRef::element_type, remove unnecessary assignment operator.

Define element_type for consistency with other classes (wxSharedPtr, weak_ptr)
and to allow accessing the type from the template code.

Also remove the apparently unnecessary template assignment operator:
assignment from an expression convertible to type T* works even without it and
while assigning something implicitly convertible to T* wouldn't work now, it's
for the best as we don't want to rely on such implicit conversions.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63933 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775