removing virtual destructor for CAN_Message;

it could be compiled using a C compiler where
the destructor will not make sense.

libraries/mbed/api/can_helper.h

@@ -41,9 +41,6 @@ struct CAN_Message {
     unsigned char  len;                // Length of data field in bytes
     CANFormat      format;             // 0 - STANDARD, 1- EXTENDED IDENTIFIER
     CANType        type;               // 0 - DATA FRAME, 1 - REMOTE FRAME
-
-    virtual ~CAN_Message() {
-    }
 };
 typedef struct CAN_Message CAN_Message;