staging:iio:gyro: Remove stale Makefile entry

Commit a301d425 ("staging:iio:gyro remove adis16251 driver as now supported by
adis16260 driver") removed the adis16251, but left its Makefile entry intact.
This patch removes the unused Makefile entry.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>