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-command/src/default_context.rs

@@ -1,7 +1,4 @@
-use crate::{
-    help::{HelpAliases, HelpCommands, HelpEscapes, HelpExterns, HelpModules, HelpOperators},
-    *,
-};
+use crate::*;
 use nu_protocol::engine::{EngineState, StateWorkingSet};
 
 pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {