Fix ignored clippy lints (#12160)

# Description
Fixes some ignored clippy lints.

# User-Facing Changes
Changes some signatures and return types to `&dyn Command` instead of
`&Box<dyn Command`, but I believe this is only an internal change.

crates/nu-command/tests/commands/table.rs

@@ -2346,7 +2346,6 @@ fn join_lines(lines: impl IntoIterator<Item = impl AsRef<str>>) -> String {
 }
 
 // util function to easier copy && paste
-#[allow(dead_code)]
 fn _print_lines(s: &str, w: usize) {
     eprintln!("{:#?}", _split_str_by_width(s, w));
 }

crates/nu-command/tests/commands/where_.rs

@@ -1,5 +1,5 @@
 use nu_test_support::nu;
-#[allow(unused_imports)]
+#[cfg(feature = "sqlite")]
 use nu_test_support::pipeline;
 
 #[test]