Remove `scanner` parameter from vulnerability_findings REST endpoint

What does this MR do?

Remove the scanner parameter from the vulnerability_findings REST API endpoint.

This endpoint is in alpha, so no deprecation notice is required. The parameter doesn't actually work either, so it won't be missed. Last but not least, we're looking to deprecate the whole REST vulnerabilities API once there's parity with GraphQL since the endpoints never made it out of alpha anyway.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #322803 (closed)