Use nightly clippy to kill dead code/fix style (#12334)

- **Remove duplicated imports**
- **Remove unused field in `CompletionOptions`**
- **Remove unused struct in `nu-table`**
- **Clarify generic bounds**
- **Simplify a subtrait bound for `ExactSizeIterator`**
- **Use `unwrap_or_default`**
- **Use `Option` directly instead of empty string**
- **Elide unneeded clone in `to html`**

crates/nu-pretty-hex/src/pretty_hex.rs

@@ -1,5 +1,4 @@
-use core::primitive::str;
-use core::{default::Default, fmt};
+use core::fmt;
 use nu_ansi_term::{Color, Style};
 
 /// Returns a one-line hexdump of `source` grouped in default format without header