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-cmd-extra/src/extra/formats/to/html.rs

@@ -680,10 +680,9 @@ fn run_regexes(hash: &HashMap<u32, (&'static str, String)>, contents: &str) -> S
     let hash_count: u32 = hash.len() as u32;
     for n in 0..hash_count {
         let value = hash.get(&n).expect("error getting hash at index");
-        //println!("{},{}", value.0, value.1);
         let re = Regex::new(value.0).expect("problem with color regex");
         let after = re.replace_all(&working_string, &value.1[..]).to_string();
-        working_string = after.clone();
+        working_string = after;
     }
     working_string
 }