IIO: ADC: stm32-dfsdm: remove unused variable again

The merge between commit abaca806 ("IIO: ADC: stm32-dfsdm: code
optimization") and commit 2353758b ("IIO: ADC: stm32-dfsdm: avoid
unused-variable warning") left one variable behind that is no longer
needed and can be removed, as shown by the gcc warning:

drivers/iio/adc/stm32-dfsdm-core.c: In function 'stm32_dfsdm_probe':
drivers/iio/adc/stm32-dfsdm-core.c:245:29: error: unused variable 'of_id' [-Werror=unused-variable]

Fixes: d84b4c7c ("Merge branch 'topic/iio' of https://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound

 into asoc-st-dfsdm")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>