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-protocol/src/pipeline_data/mod.rs

@@ -595,14 +595,14 @@ impl PipelineData {
     }
 
     /// Simplified flatmapper. For full iterator support use `.into_iter()` instead
-    pub fn flat_map<U: 'static, F>(
+    pub fn flat_map<U, F>(
         self,
         mut f: F,
         ctrlc: Option<Arc<AtomicBool>>,
     ) -> Result<PipelineData, ShellError>
     where
         Self: Sized,
-        U: IntoIterator<Item = Value>,
+        U: IntoIterator<Item = Value> + 'static,
         <U as IntoIterator>::IntoIter: 'static + Send,
         F: FnMut(Value) -> U + 'static + Send,
     {