bpo-42747: Remove Py_TPFLAGS_HAVE_AM_SEND and make Py_TPFLAGS_HAVE_VERSION_TAG no-op (GH-27260)

* Remove code that checks Py_TPFLAGS_HAVE_VERSION_TAG
    
    The field is always present in the type struct, as explained
    in the added comment.

* Remove Py_TPFLAGS_HAVE_AM_SEND
    
    The flag is not needed, and since it was added in 3.10 it can be removed now.