Remove Serial protected member functions and attributes

Author: Jenny Plunkett

drivers/Serial.h

@@ -99,13 +99,15 @@ class Serial : public SerialBase, public Stream, private NonCopyable<Serial> {
         return SerialBase::writeable();
     }
 
+#if !(DOXYGEN_ONLY)
 protected:
     virtual int _getc();
     virtual int _putc(int c);
     virtual void lock();
     virtual void unlock();
 
     PlatformMutex _mutex;
+#endif
 };
 
 } // namespace mbed