Bluetooth: crypto: Refactor a few items

After merging #51809, a few things were spotted that needed tweaking.
Among them:

- Rename the folder from bt_crypto to crypto, since the prefix is
  unnecesary
- Remove the include folder
- Remove the path from the global include paths

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>