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/conversions/into/cell_path.rs

@@ -1,5 +1,5 @@
 use nu_engine::command_prelude::*;
-use nu_protocol::ast::{CellPath, PathMember};
+use nu_protocol::ast::PathMember;
 
 #[derive(Clone)]
 pub struct IntoCellPath;

crates/nu-command/src/conversions/into/duration.rs

@@ -1,9 +1,6 @@
 use nu_engine::command_prelude::*;
 use nu_parser::{parse_unit_value, DURATION_UNIT_GROUPS};
-use nu_protocol::{
-    ast::{CellPath, Expr},
-    Unit,
-};
+use nu_protocol::{ast::Expr, Unit};
 
 const NS_PER_SEC: i64 = 1_000_000_000;
 #[derive(Clone)]