Remove legacy scale down code

[x13n]

What type of PR is this?

/kind cleanup
/kind deprecation

What this PR does / why we need it:

Legacy scale down logic didn't support draining multiple nodes simultaneously. Now that the new implementation has been stable for more than a year, it is time to drop the legacy logic.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

I had to change unit tests a bit - the legacy logic used PDBs a bit differently, so number of List calls changed. I also made a few minor improvements to improve stability of static_autoscaler_test.go.
Another notable is ScaleDownNoUnneeded scale down status removal - it was only used in the legacy implementation.

Does this PR introduce a user-facing change?

`--parallel-drain` flag was removed. To ensure only one node is drained at the same time, use `--max-drain-parallelism=1`.
`--max-empty-bulk-delete` flag was deprecated. It still works, but is equivalent to and will be replaced by `--max-scale-down-parallelism` in a future release.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[towca]

👷
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: towca, x13n

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