[#4165] Remove duplicate methods from InputInterface

- `InputInterface` had a BC break, as it added `setContinueIfEmpty()` and
  `continueIfEmpty()` -- which were technically already defined in
  `EmptyContextInterface`, which `Input` had been modified to implement.
  Removing the methods from `InputInterface` removes the BC break, and fixes an
  issue that appears in 5.3.3 with one class implementing two interfaces that
  define compatible methods.