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

[nazarhussain]

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