Excise code relating to pre-SCT embedding issuance flow

[rolandshoemaker]

1,663 deletions.

This ended up being a bit... bigger than I expected.

Things removed:

• features.EmbedSCTs (and all the associated RA/CA/ocsp-updater code etc)
• ca.enablePrecertificateFlow (and all the associated RA/CA code)
• sa.AddSCTReceipt and sa.GetSCTReceipt RPCs
• publisher.SubmitToCT and publisher.SubmitToSingleCT RPCs

Fixes #3755.

[rolandshoemaker]

I deleted the EmbedSCTs feature const, that actually needs to stay so we don't break deployed binaries that have in in their configs. Will revert that change this afternoon.

[cpu]

This is a really nice diff 😍

Beyond the deployability concern you flagged RE: the EmbedSCTs feature I think removing RPCs will need to be done in a follow-up after all the client code that called the to-be-removed RPCs is removed, no?

[cpu]

Not directly caused by this PR, but I don't understand this comment. Can it be removed/rephrased?

[rolandshoemaker]

I think it is still correct, but tbh I'm not 100% sure I understand what it's saying, so ¯\_(ツ)_/¯.

[rolandshoemaker]

Beyond the deployability concern you flagged RE: the EmbedSCTs feature I think removing RPCs will need to be done in a follow-up after all the client code that called the to-be-removed RPCs is removed, no?

None of the RPCs that are removed are currently used in any hot code paths in boulder so it should be safe to rip them all out in one fell swoop.