actions/get-merge-commit: remove push branch

This action will not be called for push events anymore. For those, the
`push.yml` workflow is run instead, which hardcodes `mergedSha`.

.github/actions/get-merge-commit/action.yml

@@ -35,8 +35,6 @@ runs:
       uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
       with:
         script: |
-          if (context.eventName == 'push') return core.setOutput('mergedSha', context.sha)
-
           for (const retryInterval of [5, 10, 20, 40, 80]) {
             console.log("Checking whether the pull request can be merged...")
             const prInfo = (await github.rest.pulls.get({