Remove unnecessary #[allow(...)] annotations (#6870)

* Remove unnecessary `#[allow]` annots

Reduce the number of lint exceptions that are not necessary with the
current state of the code (or more recent toolchain)

* Remove dead code from `FileStructure` in nu-command

* Replace `allow(unused)` with relevant feature switch

* Deal with `needless_collect` with annotations

* Change hack for needless_collect in `from json`

This change obviates the need for `allow(needless_collect)`
Removes a pessimistic allocation for empty strings, but increases
allocation size to `Value`

Probably not really worth it.

* Revert "Deal with `needless_collect` with annotations"

This reverts commit 05aca98445.

The previous state seems to better from a performance perspective as a
`Vec<String>` is lighter weight than `Vec<Value>`

crates/nu-command/src/viewers/table.rs

@@ -774,7 +774,6 @@ fn convert_to_table(
 }
 
 #[allow(clippy::too_many_arguments)]
-#[allow(clippy::into_iter_on_ref)]
 fn convert_to_table2<'a>(
     row_offset: usize,
     input: impl Iterator<Item = &'a Value> + ExactSizeIterator + Clone,