silence a clang warning -Wcovered-switch-default

There's no reason to catch DEFAULT there anyways, most likely if
someone adds a new format to the enum, we want them to specify a
new extension for it.

src/serialization/compression.hpp

@@ -31,7 +31,6 @@ namespace compression {
 		case BZIP2:
 			return ".bz2";
 		case NONE:
-		default:
 			return "";
 		}
 	}