[CICD] Remove unused variables and fix syntax

    
This commit remove variables that are not used anymore and fix yaml
syntax errors.

.gitlab-ci.yml

@@ -12,6 +12,4 @@ variables:
   # FIXME: This is only necessary due to the following GitLab limitation:
   # https://gitlab.com/gitlab-org/gitlab/-/issues/209904
   # As soon as this gets fixed upstream, the hard-coded branch name should be removed.
-  MASTER_BRANCH: seco_lf-5.10.52_2.1.0
   BB_RECIPE_NAME: imx-mkimage
-  LAYER_PROJECT_PATH: edgehog/layers/seco/meta-seco-imx
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