ci: remove ubuntu_init from rebase workflow; remove snap files

To align the build pipeline with the one for the installer
(`ubuntu-desktop-bootstrap`), we'll build the `ubuntu-desktop-init` snap
from the `snap/ubuntu-destkop-init/*` branches in the future.
This PR removes the files that are no longer needed on this branch and
removes 'init' from the soon to be deprecated rebase workflow.

.github/workflows/rebase-for-snap-cicd.yaml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-22.04
     strategy:
       matrix:
-        input: ["init", "reset-tools"]
+        input: ["reset-tools"]
     steps:
       - uses: actions/checkout@v4
         with: