Remove unused import from sys/pal/hermit/os.rs

This fixes

```
error: unused import: `str`
 --> library/std/src/sys/pal/hermit/os.rs:6:22
  |
6 | use crate::{fmt, io, str};
  |                      ^^^
  |
  = note: `-D unused-imports` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(unused_imports)]`
```

This was caused by 845311a.

Author: mkroening

library/std/src/sys/pal/hermit/os.rs

@@ -3,7 +3,7 @@ use crate::ffi::{OsStr, OsString};
 use crate::marker::PhantomData;
 use crate::path::{self, PathBuf};
 use crate::sys::unsupported;
-use crate::{fmt, io, str};
+use crate::{fmt, io};
 
 pub fn errno() -> i32 {
     unsafe { hermit_abi::get_errno() }