MONGOID-5702: Monkey Patch Removal: Remove __numeric__ class method from Numeric module

[johnnyshields]

It is now inlined into its only usage. It was previously declared @api private so assume it is safe to delete.

[johnnyshields]

This should be a straightforward merge.

[jamis]

@johnnyshields thanks for this one, yeah, it should be totally safe to apply, thanks to that private flag.

However, I feel like this is a step backward in readability. The evolve method is harder to grok at a glance. I understand wanting to remove monkeypatched methods on standard classes (three cheers for fixing that, hurray!) but I feel like there's still value in isolating the responsibility that __numeric__ currently holds.

Mongoid::Utils might be a reasonable host for such functionality, if the goal is primarily to remove that monkeypatch.