AP_HAL_Empty: Remove unused extern declaration for HALs

These are "left-overs" from how things worked before commit
"AP_HAL: make code not depend on concrete HAL
implementations". The real declaration now lives inside get_HAL() function.

Use the opportunitiy to change the files to use "#pragma once".

libraries/AP_HAL_Empty/HAL_Empty_Class.h

@@ -1,6 +1,4 @@
-
-#ifndef __AP_HAL_EMPTY_CLASS_H__
-#define __AP_HAL_EMPTY_CLASS_H__
+#pragma once
 
 #include <AP_HAL/AP_HAL.h>
 
@@ -14,8 +12,3 @@ public:
 private:
     Empty::EmptyPrivateMember *_member;
 };
-
-extern const HAL_Empty AP_HAL_Empty;
-
-#endif // __AP_HAL_EMPTY_CLASS_H__
-