adc: adc_lmp90xxx: Remove DT_INST_0_* comments

We have a number of cases that now look like:
   #if DT_HAS_COMPAT(ti_lmp90077)
   LMP90XXX_DEVICE(90077, 0, 16, 4);
   #endif /* DT_INST_0_TI_LMP90077 */

The DT_INST_0_TI_LMP90077 comment is stale, and doesn't add much since
the #if associated with the #endif is just 2 lines up.  Removing the old
comments seems the best cleanup.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>