tier-0: drop mkdir -p /run workaround

This has long been unnecessary.

See also: https://github.com/coreos/fedora-coreos-config/commit/029d320bfe95ab0c59b6fb376467c03397588e60

tier-0/bootupd.yaml

@@ -25,7 +25,5 @@ postprocess:
   - |
     #!/bin/bash
     set -xeuo pipefail
-    # Until we have https://github.com/coreos/rpm-ostree/pull/2275
-    mkdir -p /run
     # Transforms /usr/lib/ostree-boot into a bootupd-compatible update payload
     /usr/bin/bootupctl backend generate-update-metadata