Remove `SECONDS_PER_SLOT` and `ATTESTATION_SUBNET_PREFIX_BITS` after gloas

No, we should keep it until after gloas, will create a reminder issue to remove it from our code entirely, in general removing constants within a fork is not ok as it might break consumers still using them, we need a 1 fork deprecation period to allow for a smooth transition, see [Lido bringing this up here](https://discord.com/channels/595666850260713488/598292067260825641/1479417062513442828) since they use it in their oracle

_Originally posted by @nflaig in https://github.com/ChainSafe/lodestar/pull/9180#discussion_r3050425459_