Mantis: move MIXER vars to airframe init

The two were actually conflicting and are now consolidated.

ROMFS/px4fmu_common/init.d/airframes/4061_atl_mantis_edu

@@ -16,8 +16,10 @@
 
 . ${R}etc/init.d/rc.mc_defaults
 
-set MIXER none
-set MIXER_AUX none
+# We set the mixer and ESC manually.
+set MIXER skip
+set MIXER_AUX skip
+set MIXER_FILE ""
 
 # Battery settings
 param set-default BAT_CRIT_THR 0.20

boards/atl/mantis-edu/init/rc.board_defaults

@@ -26,8 +26,3 @@ set LOGGER_ARGS "-m mavlink -c 0.5"
 # Save missions RAM
 set DATAMAN_ARGS "-r"
 
-# We set the mixer and ESC manually.
-set MIXER_FILE ""
-set MIXER "skip"
-set OUTPUT_MODE "skip"
-