fix: remove duplicate declarations left by the upstream merge

The 2026-07-19 upstream merge auto-merged two additions into the same class
bodies without conflicting, producing duplicate declarations:

  examples/simple_room_server/MyMesh.h  - getCADEnabled()
  variants/heltec_v4/HeltecV4Board.h    - setLoRaFemLnaEnabled(),
                                          canControlLoRaFemLna(),
                                          isLoRaFemLnaEnabled()

Same class of breakage as the RadioLibWrapper::_cad_enabled and
MyMesh::getCADEnabled() duplicates already fixed in the merge commit. These
survived because the merge was validated with the two prescribed MQTT smoke
builds, and neither of them compiles simple_room_server or the heltec_v4
variant - so CI was the first thing to touch the broken files.

Now verified across ALL 32 observer envs locally, not a sample: 32/32 build.

examples/simple_room_server/MyMesh.h

@@ -171,9 +171,6 @@ protected:
   int getInterferenceThreshold() const override {
     return _prefs.interference_threshold;
   }
-  bool getCADEnabled() const override {
-    return _prefs.cad_enabled;
-  }
   int getAGCResetInterval() const override {
     return ((int)_prefs.agc_reset_interval) * 4000;   // milliseconds
   }

variants/heltec_v4/HeltecV4Board.h

@@ -37,8 +37,4 @@ public:
   }
   float getAdcMultiplier() const override { return adc_mult; }
   const char* getManufacturerName() const override;
-
-  bool setLoRaFemLnaEnabled(bool enable) override;
-  bool canControlLoRaFemLna() const override;
-  bool isLoRaFemLnaEnabled() const override;
 };