px4io: moving mixing to FMU side

Using mixers on the IO side had a remote benefit of being able to
override all control surfaces with a radio remote on a fixed wing.
This ended up not being used that much and since the original design
10 years ago (2011) we have been able to convince ourselves that the
overall system stability is at a level where this marginal benefit,
which is not present on multicopters, is not worth the hazzle.

Co-authored-by: Beat Küng <beat-kueng@gmx.net>
Co-authored-by: Daniel Agar <daniel@agar.ca>

src/lib/systemlib/system_params.c

@@ -83,22 +83,6 @@ PARAM_DEFINE_INT32(SYS_AUTOCONFIG, 0);
  */
 PARAM_DEFINE_INT32(SYS_HITL, 0);
 
-/**
- * Set restart type
- *
- * Set by px4io to indicate type of restart
- *
- * @min 0
- * @max 2
- * @value 0 Data survives resets
- * @value 1 Data survives in-flight resets only
- * @value 2 Data does not survive reset
- * @category system
- * @volatile
- * @group System
- */
-PARAM_DEFINE_INT32(SYS_RESTART_TYPE, 2);
-
 /**
  * Set multicopter estimator group
  *