AP_RPM: removed create() method for objects

See discussion here:

  https://github.com/ArduPilot/ardupilot/issues/7331

we were getting some uninitialised variables. While it only showed up in
AP_SbusOut, it means we can't be sure it won't happen on other objects,
so safest to remove the approach

Thanks to assistance from Lucas, Peter and Francisco

libraries/AP_RPM/AP_RPM.h

@@ -29,9 +29,7 @@ class AP_RPM
     friend class AP_RPM_Backend;
 
 public:
-    static AP_RPM create() { return AP_RPM{}; }
-
-    constexpr AP_RPM(AP_RPM &&other) = default;
+    AP_RPM();
 
     /* Do not allow copies */
     AP_RPM(const AP_RPM &other) = delete;
@@ -95,8 +93,6 @@ public:
     bool enabled(uint8_t instance) const;
 
 private:
-    AP_RPM();
-
     RPM_State state[RPM_MAX_INSTANCES];
     AP_RPM_Backend *drivers[RPM_MAX_INSTANCES];
     uint8_t num_instances:2;

libraries/AP_RPM/examples/RPM_generic/RPM_generic.cpp

@@ -26,7 +26,7 @@ void loop();
 
 const AP_HAL::HAL& hal = AP_HAL::get_HAL();
 
-static AP_RPM RPM = AP_RPM::create();
+static AP_RPM RPM;
 
 char sensor_state;