media: i2c: imx290: Do not reset exposure time from set_fmt

V4L2 controls are meant to be persistent, and only altered if
the values break hardware constraints.

In imx290_set_fmt the CID_VBLANK parameter was being reset to
the mode default, and therefore the maximum exposure value
potentially also changed.

VBLANK is no longer reset, and even if it is, then the set_ctrl
that will be called will alter the range for the exposure
control. Therefore there is no need to reset the exposure time
from set_fmt.

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