staging: most: i2c: remove unnecessary poison_channel call

This removes call of the poison_channel that is:
  - not allowed after most_deregister_interface;
  - is made during the most_deregister_interface call.

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>