i2c: Drop unused RTC driver IDs

The x1208, pcf8563 and isl1208 RTC drivers have been converted to
new-style i2c drivers, so they no longer use I2C driver IDs.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>