Remove Config UI support of SAST_DEFAULT_ANALYZERS [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

As part of #290777 (closed), we are removing support for the SAST_DEFAULT_ANALYZERS CI variable from the SAST Config UI.

Customers will see no change in the behavior of the SAST Config UI. And we were previously writing SAST_EXCLUDED_ANALYZERS to the resulting MR, so the only change is that the value of the SAST_DEFAULT_ANALYZERS value will no longer be taken into consideration when determining the value to write for SAST_EXCLUDED_ANALYZERS.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have added/updated documentation, or it's not needed. (Is documentation required?)
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• 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.)
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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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