chore: Remove unused `base16` dependency and dead `hex` wrapper function (#8207)
### Description
Noticed these dependencies/functions were unused.
For the most part, it's better to do these conversions with the built-in
`format!()`.
Parsing base16 could make sense for this as a dependency, but we're not
doing that.
### Testing Instructions
```
cargo check
```
Also grepped through next.js to check that we're not using it there.