i2c-pxa: Drop leftover comment

Commit 488bf314 ("i2c: Allow i2c_add_numbered_adapter() to assign a
bus id") reworked i2c-pxa driver leaving obsolete comment.

This commit simply drops it.

Signed-off-by: Karol Lewandowski <k.lewandowsk@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>