base: remove diagnostic message in destructor

The LOG message in the Signal_handler_thread's destructor is just an
diagnostic leftover.

Fixes #3666.

repos/base/src/lib/base/signal.cc

@@ -70,7 +70,6 @@ class Signal_handler_thread : Thread, Lock
 
 		~Signal_handler_thread()
 		{
-			log("~Signal_handler_thread");
 			env_deprecated()->pd_session()->free_signal_source(_signal_source->rpc_cap());
 		}
 };