Simplify SAST component to match CI template

What does this MR do and why?

Simplifies the SAST component logic, introduced by Upgrade to latest rules (!27 - merged) • Klok, to match the semgrep-sast job from the SAST.gitlab-ci.yml template.

Manual testing

When a single *.php file exists:

• and run_advanced_sast: true is set:

  Both the gitlab-advanced-sast and semgrep-sast jobs are present: pipeline.

  note: in this case, the ideal behaviour is to only run a single gitlab-advanced-sast job, and the semgrep-sast job should not be present, however, due to the concerns pointed out in Refactor php-related SAST rules (gitlab-org/gitlab#559997 - closed) • Adam Cohen • 18.4, we've decided to implement this behaviour for now.

• and run_advanced_sast: false is set:

  Only a single semgrep-sast job is present: pipeline

/cc @klokkenman1 @thiagocsf