Stop adding the `isDefaultController` value to `$routeData` in RequestHandler

Stop adding the `isDefaultController` value to `$routeData` in RequestHandler

The `$routeData` array is only processed locally, except when it is passed to
LinkHandler for renamed controllers (which can't happen for default
controllers), thus this is completely redundant with simply checking
`->isDefaultController()` in the other places, with the latter being much more
clear, because one does not need to think about where the `$routeData` might
end up.