Attach the css for the admin form in the render array

Problem/Motivation

The css file for the admin form is attached in the hook_preprocess_page() hook but this is not neccesary because can be added directly in the form render array.

Proposed resolution

Attach the css file in the admin form render array.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.