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/dataframe/eager/columns.rs

@@ -55,7 +55,6 @@ impl Command for ColumnsDF {
     }
 }
 
-#[allow(clippy::needless_collect)]
 fn command(
     _engine_state: &EngineState,
     _stack: &mut Stack,

crates/nu-command/src/dataframe/eager/dtypes.rs

@@ -56,7 +56,6 @@ impl Command for DataTypes {
     }
 }
 
-#[allow(clippy::needless_collect)]
 fn command(
     _engine_state: &EngineState,
     _stack: &mut Stack,