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.

lodestar/packages/beacon-node/src/api/impl/validator/index.ts

Lines 475 to 481 in e5dcc96

	// At any point either the builder or execution or both flows should be active.
	//
	// Ideally such a scenario should be prevented on startup, but proposerSettingsFile or keymanager
	// configurations could cause a validator pubkey to have builder disabled with builder selection builder only
	// (TODO: independently make sure such an options update is not successful for a validator pubkey)
	//
	// So if builder is disabled ignore builder selection of builderonly if caused by user mistake

worth making an issue to follow up

Originally posted by @wemeetagain in #6241 (comment)