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-json/src/value.rs

@@ -2,7 +2,7 @@
 use std::collections::{btree_map, BTreeMap};
 
 #[cfg(feature = "preserve_order")]
-use linked_hash_map::{self, LinkedHashMap};
+use linked_hash_map::LinkedHashMap;
 
 use std::fmt;
 use std::io;
@@ -1094,9 +1094,9 @@ impl<'de> de::MapAccess<'de> for MapDeserializer {
     }
 }
 
-pub fn to_value<T: ?Sized>(value: &T) -> Result<Value>
+pub fn to_value<T>(value: &T) -> Result<Value>
 where
-    T: ser::Serialize,
+    T: ser::Serialize + ?Sized,
 {
     value.serialize(Serializer)
 }