l2tp: remove BUG_ON refcount value in l2tp_session_free

l2tp_session_free is only called by l2tp_session_dec_refcount when the
reference count reaches zero, so it's of limited value to validate the
reference count value in l2tp_session_free itself.

Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>