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/filter/LoadChain.cxx

@@ -42,6 +42,8 @@ filter_chain_append_new(PreparedFilter &chain, const char *template_name)
 		throw FormatRuntimeError("Filter template not found: %s",
 					 template_name);
 
+	cfg->SetUsed();
+
 	// Instantiate one of those filter plugins with the template name as a hint
 	auto f = filter_configured_new(*cfg);