FW autotrim: replace servo_autotrim_iterm_rate_limit setting with hardcoded constant

The configurable CLI setting added in #11215 was based on a default of 2
with no empirical basis. Blackbox analysis of real flight data shows i-term
rates of 4–9 units/s (median) and up to 30 units/s (90th percentile) during
settled cruise flight. A hardcoded constant of 30 gives reasonable protection
against turbulence-driven i-term spikes without exposing a tuning knob that
pilots cannot meaningfully calibrate.

Author: sensei-hacker

src/main/flight/servos.c

@@ -603,6 +603,7 @@ void processServoAutotrimMode(void)
     }
 }
 
+
 void processContinuousServoAutotrim(const float dT)
 {
     static timeMs_t lastUpdateTimeMs;