Partial revert for checkout accessories to non user

[snipe]

This removes the location and asset selector introduced in #15185, since the other end of that (where you can see the accessories that are checked out to the location/asset on the location/asset's page, which would make for a very confusing user experience.

I have a WIP to fix that here, but this at least reverts the front end of it so we don't confuse users and aren't holding up develop.

[what-the-diff]

PR Summary

• Enhancements in CheckInOutRequest.php
  The update makes sure that the User assigned in the request is returned as default. This improves the system’s consistency, ensuring that the correct User associated at first place is consistently used throughout.

• Simplification of checkout.blade.php
  The code removals essentially make the page less cluttered, removing unnecessary forms for object selection (checkout-selector, asset-select, location-select). Instead, a user-selection form (edit.user-select) has been added with a mandatory field which simplifies the user experience, limiting to only necessary fields.

• Styling and validation improvements in checkout.blade.php
  Unnecessary style and attribute related codes were removed for various forms such as edit.asset-select and edit.location-select, making the code cleaner. The styling associated with the hidden form location-select was also eliminated. Lastly, validations were enhanced by adding an error class in the form group div class whenever there are validation errors, thus making error handling more seamless and user-friendly.