ci-automation: remove Equinix Metal testing

[tormath1]

After years of good services and sponsoring from Equinix Metal, we need to retire those tests for AMD64 / ARM64 bare metal instances.

As Equinix Metal offering is still around until June 2026, let's continue to produce and release Flatcar images for any users still using those images but we won't test them as the Flatcar account won't be covered by sponsoring.

[krnowak]

I think we should drop EM from Jenkins first, no?

[tormath1]

Nighlies have passed with EM dropped from Jenkins. Let's merge this.

Cherry-picked to:

• flatcar-4515
• flatcar-4459
• flatcar-4081