Clean up post EIP-8282

[ensi321]

We need to remove all references to the builder voluntary exit mechanism that is now removed. Eg. remove getBuilderVoluntaryExitSignatureSet and merge getVoluntaryExitsSignatureSets and getVoluntaryExitSignatureSet into one.

Could be a future refactoring opportunity

yes there is more stuff we might wanna clean up, eg. there is no need to validate pending deposits anymore so we can probably remove the cache or at least keep it only for the fork transition, this branch still needs to be cleaned up carefully

Originally posted by @nflaig in #9541 (comment)