Remove bonus inflection in _setModelClass()

This inflection was causing issues in controllers that had plural names.
For example NavigationMenusController. I've also axed automatically
setting modelClass on shell classes. Many shells don't need the
conventional model. By defining a modelClass property shells can opt-in
to having a model initialized for free.

Fixes #3629
Merge pull request #3633 from markstory/issue-3629-extra-inflection

Remove bonus inflection in _setModelClass()