Revert "build: specify overlays for each org (#13)"

[mortennordseth]

This reverts commit 17323c6. As we are pushing for all tenants at the same time I am in the process of moving the kustomization to base and we won't need to specify overlays (https://github.com/AtB-AS/gcp-infra/pull/419).

For future references, it's not necessary to use a matrix to specify the push and release overlays. It can be done so:

with:
      image: gcr.io/atb-mobility-platform/planner-web
      push_overlay: atb-staging-c420 nfk-staging-dac7 fram-staging-dc00
      release_overlay: atb-prod-2e7e nfk-prod-ce66 fram-prod-019b
      overlays_base: manifests/amp/overlays