i2c: designware: Remove unused completion code from i2c-designware-slave

Remove unused completion code from i2c-designware-slave.c. Used only in
i2c-designware-master.c.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>