ds2782_battery: Fix clientdata on removal

Probably due to a copy & paste bug, clientdata was set again to the data
structure (which is freed immediately afterwards) when it should be
NULLed. Just remove the calls as the i2c-core does this automatically
now.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Ryan Mallon <ryan@bluewatersys.com>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>