cdc-acm: remove dead code

The wait_event_interruptible_timeout in acm_port_down is
never reached. Remove it to avoid possible deadlocks
with the big tty mutex if someone were to start using
the blocking version of acm_port_down.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>