Remove OnDemandScansApp component

What does this MR do?

Removes the OnDemandScansApp component which used to be the entry point for On-demand scans but was eventually replaced with OnDemandScansForm.

This follows-up on !50721 (diffs, comment 478518366), where we decided to render OnDemandScansForm directly.

Screenshots (strongly suggested)

No visual change as the component wasn't used anywhere anymore.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is a backstage change.

Related to #296823 (closed)