Remove old DynamicServoFactory which reads from saltfs

[aneeshusa]

Now that the new dynamic factory is working (which reads the steps
configuration from the main servo repo), remove the old one.
However, re-use the old name (DynamicServoFactory instead of
DynamicServoYAMLFactory) for concision and clarity.

Fixes #316. Blocked on servo/servo#13611.

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This change is

[aneeshusa]

r? @edunham

[aneeshusa]

FYI I did this at 2am, so I'd like to take another look in the morning :)

[aneeshusa]

servo/servo#13611 has merged, so this is no longer blocked.

[aneeshusa]

steps.yml was changed, now blocked on a follow-up PR.

[bors-servo]

☔ The latest upstream changes (presumably #503) made this pull request unmergeable. Please resolve the merge conflicts.

[jdm]

The followup PR merged. It would be really great to get this reviewed and throw the switch before we change steps.yml again.

[larsbergstrom]

This looks good. I think we can land this (even though it'll bitrot my MSVC PR). @aneeshusa Is this good to go?

My only worry is that now to add/change builders we have to do coordinated commits AND buildbot restarts across servo/servo and servo/saltfs and that's certainly going to cause breakage every time we have to do it.

[aneeshusa]

Yes, this should be ready. This will optimize for the common case (changing steps for pre-existing builders), and I don't think having to make an extra PR to add/remove a builder (less common) is too onerous.

[larsbergstrom]

@bors-servo r+

[bors-servo]

📌 Commit 4075ac4 has been approved by larsbergstrom

[bors-servo]

⚡ Test exempted - status

[larsbergstrom]

This has been deployed.

[larsbergstrom]

Oh, hrm, should we have removed steps.yml from saltfs, too?

[aneeshusa]

I made #513 for that, good call.

[larsbergstrom]

What's the easiest way to back this out via salt? The salt-run stuff that I did to stage the new mac?

[aneeshusa]

If #514 doesn't work, easiest is probably to git revert the commits from before this PR up to current master, run a regular highstate, and prepare another PR (with a test builder on Windows!) at our leisure.

The other option is using the override folder and checking out an old git revision, notes at https://github.com/servo/saltfs/blob/b64ff04d219d1fe34c653ae5939f43fc4311d30b/salt/files/master/ADMIN_README.