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-parser/src/parser.rs

@@ -1604,7 +1604,6 @@ pub fn parse_string_interpolation(
 
     let mut b = start;
 
-    #[allow(clippy::needless_range_loop)]
     while b != end {
         if contents[b - start] == b'('
             && (if double_quote && (b - start) > 0 {