Fix Security Scanning setup

According to [1],

> Secret Detection jobs `secret_detection_default_branch` and
> `secret_detection` were consolidated into one job,
> `secret_detection`."

so the setup in `.gitlab-ci.yaml` for `secret_detection_default_branch`
is obsolete.

[1]: https://docs.gitlab.com/ee/user/application_security/secret_detection/#configuration

Signed-off-by: Patrick Dohmen <dl4pd@darc.de>

.gitlab-ci.yml

@@ -322,11 +322,6 @@ secret_detection:
   needs:
     - job: api
       artifacts: true
-secret_detection_default_branch:
-  stage: security
-  needs:
-    - job: api
-      artifacts: true
 license_scanning:
   stage: security
   needs: