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>

drivers/staging/iio/gyro/Makefile

@@ -14,8 +14,5 @@ obj-$(CONFIG_ADIS16130) += adis16130.o
 adis16260-y             := adis16260_core.o
 obj-$(CONFIG_ADIS16260) += adis16260.o
 
-adis16251-y             := adis16251_core.o
-obj-$(CONFIG_ADIS16251) += adis16251.o
-
 adxrs450-y             := adxrs450_core.o
 obj-$(CONFIG_ADXRS450) += adxrs450.o