Remove outdated core and compat options

[bertdeblock]

I couldn't find these options in the corresponding interfaces or throughout
the rest of the codebase, so I assumed these might be leftovers?

[bertdeblock]

Not 100% sure about this one, but it isn't defined in the interface above and it's also a compat option.

[ef4]

Thanks, I hadn't even noticed that these were lingering.

For those following along at home:

• optionalComponents is covered instead by safeToIgnore inside packageRules.
• packageRules is still fully-supported, it's just that it's an option to @embroider/compat, not @embroider/core. You're almost certainly using compat anyway.
• splitControllers and splitRouteClasses are subsumed by splitAtRoutes, which necessary must handle controllers, route classes, and route templates together.