ringbuf: Remove unused #[used] we weren't using (#2170)

This commit removes one more instance of the `#[used]` attribute in
`ringbuf` that we don't actually need --- in the macro that generates
`counted_ringbuf!`s when counters are enabled but the actual ringbuf
isn't. I had somehow forgotten this in #2167, my bad!

Author: hawkw

lib/ringbuf/src/lib.rs

@@ -381,15 +381,13 @@ macro_rules! counted_ringbuf {
 #[macro_export]
 macro_rules! counted_ringbuf {
     ($name:ident, $t:ident, $n:expr, $init:expr, no_dedup) => {
-        #[used]
         static $name: $crate::CountedRingbuf<$t, (), $n> =
             $crate::CountedRingbuf {
                 counters: <$t as $crate::Count>::NEW_COUNTERS,
                 _c: core::marker::PhantomData,
             };
     };
     ($name:ident, $t:ident, $n:expr, $init:expr) => {
-        #[used]
         static $name: $crate::CountedRingbuf<$t, u16, $n> =
             $crate::CountedRingbuf {
                 counters: <$t as $crate::Count>::NEW_COUNTERS,