runner-clean: drop "Register qemu binfmt handlers" step

[igorpecovnik]

What

Removes the Register qemu binfmt handlers step from runner-clean/action.yml.

Why

The step ran on every job to keep qemu-user current via apt so cross-arch (amd64 host → arm64 target) rootfs postinsts could run under qemu-aarch64-static. It doesn't achieve that and is pure overhead:

• Can't fix the real failure. The distro qemu is frozen too old for newer guests — Ubuntu noble ships qemu-user-static 8.2.2, so the linux-headers deb tar postinst dies with tar: … Cannot open: Function not implemented. apt-get --only-upgrade is a no-op (already the newest version).
• Redundant with the build. lib/functions/rootfs/qemu-static.sh (prepare_host_binfmt_qemu_cross) already mounts binfmt_misc and runs update-binfmts --enable itself, provided the package is installed.
• Noisy and racy. Running apt every job loses the dpkg lock to concurrent apt (Could not get lock … held by process …); the || true then swallowed the failure and gave false confidence.

Seen in the wild (run 30941608953): 21 image builds failed on exactly this Function not implemented tar error across the x86 pool.

Where it's handled instead

At the host level, not per-job:

• Build hosts are being bumped to a newer release (resolute), which brings a current qemu able to emulate the guest.
• Runner provisioning installs qemu-user-static + binfmt-support; the build re-enables the handlers per run.

Safety

Current fleet already has qemu-user-static installed and the build re-registers binfmt each run, so no active runner regresses. YAML validated (yaml.safe_load OK).