Remove unused dependencies (#6945)

* Remove unused dependencies

Inspired by #6938 ran `cargo +nightly udeps --features extra`.
Removes 2 crates and should remove an unnecessary intra-workspace
dependency which might open up further opportunities for compilation.

* Make windows-only dependency conditional in toml

`omnipath` is only used on Windows and already behind a `#[cfg]` block
in the code. Made the dependency in `Cargo.toml` conditional as well.

* Make `nu-pretty-hex` example a proper example

This allows making `rand` a dev-dependency in this crate.

crates/nu-pretty-hex/Cargo.toml

@@ -12,17 +12,10 @@ doctest = false
 name = "nu_pretty_hex"
 path = "src/lib.rs"
 
-[[bin]]
-name = "nu_pretty_hex"
-path = "src/main.rs"
-
 [dependencies]
 nu-ansi-term = "0.46.0"
-rand = "0.8.3"
 
 [dev-dependencies]
 heapless = { version = "0.7.8", default-features = false }
+rand = "0.8.3"
 
-# [features]
-# default = ["alloc"]
-# alloc = []