fmt: remove unnecessary lint suppression

Author: tamird

src/libcore/fmt/mod.rs

@@ -322,7 +322,6 @@ impl<'a> ArgumentV1<'a> {
 
 // flags available in the v1 format of format_args
 #[derive(Copy, Clone)]
-#[allow(dead_code)] // SignMinus isn't currently used
 enum FlagV1 { SignPlus, SignMinus, Alternate, SignAwareZeroPad, }
 
 impl<'a> Arguments<'a> {