Remove undefined declaration (#4652)

The getNMEA method was introduced to the header but never defined
in code. As it's unused, remove it.

src/gps/GPS.h

@@ -297,7 +297,6 @@ class GPS : private concurrency::OSThread
     virtual int32_t runOnce() override;
 
     // Get GNSS model
-    String getNMEA();
     GnssModel_t probe(int serialSpeed);
 
     // delay counter to allow more sats before fixed position stops GPS thread