Remove old scanner filter and only use new filter on vuln report page [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

On the vulnerability report page, we have a dropdown to filter the vulnerability list by scanner type:

Previously, we were using a feature flag to toggle between the old version of this dropdown, which used the StandardFilter component, and a new version, which uses the ScannerFilter component. This feature flag was added so that we can test the performance of the new dropdown with the much larger dataset on production, and then be able to switch back to the old version if there were any issues.

Since we've verified that the new ScannerFilter component does not have performance issues, we no longer need the feature flag, nor the code that toggles between the old and new component. This MR removes that code and uses the new ScannerFilter code exclusively.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.