Windows: fix warnings in portable-pty

[rgwood]

Hi! Tiny PR, just fixes up a few build warnings when building portable-pty on Windows:

warning: the item `Path` is imported redundantly
   --> pty\src\cmdbuilder.rs:608:13
    |
9   | use std::path::{Component, Path};
    |                            ---- the item `Path` is already imported here
...
608 |         use std::path::Path;
    |             ^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_imports)]` on by default

warning: function `is_cwd_relative_path` is never used
   --> pty\src\cmdbuilder.rs:749:4
    |
749 | fn is_cwd_relative_path<P: AsRef<Path>>(p: P) -> bool {
    |    ^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default

I'm hoping to contribute more substantial Windows fixes soon. I've been noticing that every few days WezTerm pegs a CPU core on Windows but haven't been able to pinpoint why. I now have a build with debug symbols and I'll profile it next time the issue occurs.

[wez]

Thanks!