iio: ad7949: kill pointless "readback"-handling code

commit c270bbf7 upstream.

The device could be configured to spit out also the configuration word
while reading the AD result value (in the same SPI xfer) - this is called
"readback" in the device datasheet.

The driver checks if readback is enabled and it eventually adjusts the SPI
xfer length and it applies proper shifts to still get the data, discarding
the configuration word.

The readback option is actually never enabled (the driver disables it), so
the said checks do not serve for any purpose.

Since enabling the readback option seems not to provide any advantage (the
driver entirely sets the configuration word without relying on any default
value), just kill the said, unused, code.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>