Remove cluster image scanning from the options as part of vulnerability rule

Why are we doing this work

#334935 (closed) added all security scanners as part of vulnerability rules under merge request approval. However cluster image scanning doesn't apply for the same use case, therefore it has to be removed.

Relevant links

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

• backend Remove cluster_image_scanning by not using the original list of scanners.
• backend Update the DB for cases where cluster_image_scanning is present
  or
  frontend Filter it out when editing the rules.
• frontend Remove cluster_image_scanning by not using the original list of scanners.