tamer: xir (XmlWriter)[write_new]: Correct #[must_use] declaration

The return value has no meaningful side-effects at all; the write operation
failing isn't worth pointing out, since it has to be used regardless.

The normal `write` does have useful side-effects, of course.

tamer/src/ir/xir/writer.rs

@@ -142,7 +142,7 @@ pub trait XmlWriter: Sized {
     /// This is intended primarily for testing;
     ///   it is recommended that you use [`write`](XmlWriter::write) instead,
     ///     unless you _really_ need a new owned `Vec<u8>`.
-    #[must_use = "Write operation may fail"]
+    #[must_use]
     fn write_new(
         self,
         prev_state: WriterState,