refactor(shell): Simplify some code

This is from some review feedback left on rust-lang#15917 which was merged
in rust-lang#15920.

Author: epage

src/cargo/core/shell.rs

@@ -418,7 +418,7 @@ impl Shell {
             .unwrap_or(annotate_snippets::renderer::DEFAULT_TERM_WIDTH);
         let rendered = Renderer::styled().term_width(term_width).render(report);
         self.err().write_all(rendered.as_bytes())?;
-        self.err().write_all("\n".as_bytes())?;
+        self.err().write_all(b"\n")?;
         Ok(())
     }
 }