Do not allow disabling builder if builder selection is set to builder only

We need to make sure if a `proposerSettingsFile` or `keymanager` have settings for a public key to `BuilderOnly` builder selection, then we should not allow user to disable the builder for that validator.

https://github.com/ChainSafe/lodestar/blob/e5dcc963deb276f4011baa3c5b0614a1afabe7e0/packages/beacon-node/src/api/impl/validator/index.ts#L475-L481

worth making an issue to follow up

_Originally posted by @wemeetagain in https://github.com/ChainSafe/lodestar/pull/6241#discussion_r1459310256_

Start at packages/beacon-node/src/api/impl/validator/index.ts around lines 475-481 and trace how proposerSettingsFile and keymanager settings determine BuilderOnly selection. Check the validator disable path and identify the expected guard for validators configured as BuilderOnly. Done means disabling the builder is rejected for those validators while remaining allowed for others.

Written by the indexing model from the issue text.