APMrover2: remove useless variable

APMrover2/failsafe.cpp

@@ -42,8 +42,7 @@ void Rover::failsafe_check()
         // pass RC inputs to outputs every 20ms
         last_timestamp = tnow;
         hal.rcin->clear_overrides();
-        uint8_t start_ch = 0;
-        for (uint8_t ch=start_ch; ch < 4; ch++) {
+        for (uint8_t ch = 0; ch < 4; ch++) {
             hal.rcout->write(ch, hal.rcin->read(ch));
         }
         SRV_Channels::copy_radio_in_out(SRV_Channel::k_manual, true);