media: i2c: imx290: Drop incorrect comment about pixelrate

commit b5eca6fd ("media: i2c: imx290: Fix the pixel rate at 148.5Mpix/s")
corrected imx290_calc_pixel_rate to always return the correct
pixle rate of 148.5Mpix/s, but didn't remove the comment that said
it relied on some other parameters.

Drop that comment.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>