af_iucv: remove duplicate sock_set_flag

Remove duplicate sock_set_flag(sk, SOCK_ZAPPED) in iucv_sock_close,
which has been overlooked in September-commit
7514bab0.

Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>