Remove registration-only runners

[toast-gear]

Description
Since GitHub changed their routing logic to not fail pipelines that target a runs-on label that doesn't have a runner registered these are no longer needed

This work will also close some bugs linked with these pods:

• bug: registration-only pods are created with invalid name lengths #595

[FearlessHyena]

I'm not 100% sure but since GitHub Enterprise Server usually gets updated a few months later (compared to the Cloud version) it might still be using the old routing logic

[toast-gear]

#470 (comment) I've just asked the Actions product owner in an old issue, not sure if he'll see it but worth a punt.

EDIT https://docs.github.com/en/enterprise-cloud@latest/actions/hosting-your-own-runners/using-self-hosted-runners-in-a-workflow#routing-precedence-for-self-hosted-runners you can select the Article version in the top right. When 3.3 comes out we should check the routing precedence again to see if the new logic made it to 3.3

EDIT We probably need to keep this logic for legacy setups, once 3.3 is out confirm via the documentation if the routing logic has changed and update the logic to use registration runners only on old GHES envs

EDIT https://docs.github.com/en/enterprise-server@3.3/actions/hosting-your-own-runners/using-self-hosted-runners-in-a-workflow#routing-precedence-for-self-hosted-runners the new routing logic is in the 3.3 docs, looks like we only registration runners for GitHub Server versions below 3.3 now

[toast-gear]

We are going to drop support for GHES below v3.3 and so the registration runners are no longer requried by any environment.