bpo-40297: Fix test_socket.CANTest.testSendFrame (GH-19548)

The address tuple for CAN_RAW no longer returns the address family
after the introduction of CAN ISO-TP support in a30f6d45ac3. However,
updating test_socket.CANTest.testSendFrame was missed as part of the
change, so the test incorrectly attempts to index past the last tuple
item to retrieve the address family.

This removes the now-redundant check for equality against socket.AF_CAN,
as the tuple will not contain the address family.