output/wasapi: remove broken Drain() implementation

The current Drain() implementation does what Cancel() should do; it
does not wait for completion, but instead discards the buffer.

src/output/plugins/wasapi/WasapiOutputPlugin.cxx

@@ -661,7 +661,8 @@ WasapiOutput::Drain()
 {
 	assert(thread);
 
-	thread->spsc_buffer.consume_all([](auto &&) {});
+	// TODO implement
+
 	thread->CheckException();
 }