Fixes pure virtual call exception in VirtualCanDriver<>::handleRxFrame()

Stack trace:
Thread [1] (Suspended: Breakpoint hit.)
        12 __cxa_pure_virtual() libxx_cxapurevirtual.cxx:66 0x0808ca42
        11 notifyRxFrameListener() uc_dispatcher.cpp:216 0x080787ce
        10 uavcan::Dispatcher::spinOnce() uc_dispatcher.cpp:276 0x080787ce
        9 uavcan::Scheduler::spinOnce() uc_scheduler.cpp:196 0x0807b4e0
        8 spinOnce() abstract_node.hpp:88 0x080659fc
        7 spinOnce() node.hpp:132 0x080659fc
        6 UavcanNode::node_spin_once() uavcan_main.cpp:428 0x080659fc
        5 UavcanNode::run() uavcan_main.cpp:542 0x08065e74
        4 operator() uavcan_main.cpp:343 0x0806626a
        3 UavcanNode::__lambda0::_FUN() uavcan_main.cpp:343 0x0806626a
        2 task_start() task_start.c:138 0x08087720
        1 <symbol is not available> 0x00000000

src/modules/uavcan/uavcan_servers.cpp

@@ -98,6 +98,7 @@ UavcanServers::~UavcanServers()
 	if (_mutex_inited) {
 		(void)Lock::deinit(_subnode_mutex);
 	}
+	_main_node.getDispatcher().removeRxFrameListener();
 }
 
 int UavcanServers::stop(void)