Revert "ci: add action for STACKIT login"

This reverts commit bd17f455c6cc3984b91cc4d50ca88859cdb260a7.

.github/actions/login_stackit/action.yml

@@ -1,16 +0,0 @@
-name: STACKIT login
-description: "Login to STACKIT"
-inputs:
-  serviceAccountToken:
-    description: "Credentials authorized to create Constellation on STACKIT."
-    required: true
-runs:
-  using: "composite"
-  steps:
-   - name: Login to STACKIT
-     env:
-       UAT: ${{ inputs.serviceAccountToken }}
-     shell: bash
-     run: |
-        mkdir -p ~/.stackit
-        echo "${UAT}" > ~/.stackit/credentials.json