PX4 System:Expunge the nuttx adc structure from the system

   This PR is preliminary ground work for FMUv5.

   PX4 does not use the NuttX adc driver. But used the same format
   for the data returned by the nuttx ADC driver.

   There was a fixme:in src/platforms/px4_adc.h "this needs to be
   a px4_adc_msg_s type" With this PR the need for
   src/platforms/px4_adc.h goes away as the driver drv_adc.h now
   describes the px4_adc_msg_t.