drivers: adc: stm32h7: remove oversampling workaround

Removes a workaround that was required while there was a bug in
upstream STM32 HAL drivers where the oversampling was not set
correctly in all cases. This has since been fixed, and also
added to the Zephyr STM32 HAL.

The upstream issue:
https://github.com/STMicroelectronics/STM32CubeH7/issues/177



Signed-off-by: Hein Wessels <heinwessels93@gmail.com>