Drop a redundant closure

Fixes a clippy warning:

```
warning: redundant closure
  --> src/parse/mod.rs:70:9
   |
70 |         |s| unescape_quoted(s),
   |         ^^^^^^^^^^^^^^^^^^^^^^ help: replace the closure with the function itself: `unescape_quoted`
   |
   = note: `#[warn(clippy::redundant_closure)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
```

src/parse/mod.rs

@@ -67,7 +67,7 @@ pub fn Quoted_string(input: &[u8]) -> IResult<&[u8], Cow<'_, str>> {
             map_res(recognize(many0(QcontentSMTP)), std::str::from_utf8),
             DQUOTE,
         ),
-        |s| unescape_quoted(s),
+        unescape_quoted,
     )(input)
 }