config/Global: remove ConfigBlock::SetUsed() call, let caller do that

This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

src/input/Init.cxx

@@ -53,6 +53,8 @@ input_stream_global_init(EventLoop &event_loop)
 			/* the plugin is disabled in mpd.conf */
 			continue;
 
+		block->SetUsed();
+
 		try {
 			if (plugin->init != nullptr)
 				plugin->init(event_loop, *block);