docs: remove --scanners none

[nikpivkin]

Description

See #5182.

There is still a mention in the documentation of an invalid none argument for the scanners flag.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[nikpivkin]

@knqyf263 These types associated with the none scanner are not used anywhere:

• https://github.com/aquasecurity/trivy/blob/main/pkg/types/target.go#L30
• https://github.com/aquasecurity/trivy/blob/main/pkg/types/target.go#L48-L71