Clean up post EIP-8282

> 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 https://github.com/ChainSafe/lodestar/issues/9541#issuecomment-4776788126_

Start by searching the Lodestar repository for getBuilderVoluntaryExitSignatureSet, getVoluntaryExitsSignatureSets, getVoluntaryExitSignatureSet, and pending-deposit validation or cache references. Trace the fork-transition paths before changing shared logic. Done means obsolete builder voluntary-exit references are removed, signature-set handling is consolidated, and pending-deposit caching is retained only where still needed.

Written by the indexing model from the issue text.