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-cli/src/completions/completion_options.rs

@@ -95,7 +95,6 @@ impl std::error::Error for InvalidMatchAlgorithm {}
 pub struct CompletionOptions {
     pub case_sensitive: bool,
     pub positional: bool,
-    pub sort_by: SortBy,
     pub match_algorithm: MatchAlgorithm,
 }
 
@@ -104,7 +103,6 @@ impl Default for CompletionOptions {
         Self {
             case_sensitive: true,
             positional: true,
-            sort_by: SortBy::Ascending,
             match_algorithm: MatchAlgorithm::Prefix,
         }
     }

crates/nu-cli/src/completions/custom_completions.rs

@@ -108,11 +108,6 @@ impl Completer for CustomCompletion {
                                     .get("positional")
                                     .and_then(|val| val.as_bool().ok())
                                     .unwrap_or(true),
-                                sort_by: if should_sort {
-                                    SortBy::Ascending
-                                } else {
-                                    SortBy::None
-                                },
                                 match_algorithm: match options.get("completion_algorithm") {
                                     Some(option) => option
                                         .coerce_string()