gigaset: remove unnecessary poll method

The N_GIGASET_M101 line discipline implemented by the ser_gigaset
driver does not transfer any data from/to userspace through the
tty interface. Therefore a poll method is not needed.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>