refactor($animate): use $animate.$$setClassImmediately to save code

for what it's worth,  `add/removeClassImmediately` both have good reasons to exist (mainly, they exist for use in tests, and so that ngAnimate can share them without them being exposed on the `angular` object) --- but `setClassImmediately` doesn't really have a good reason to exist. It's a private API that users should not use (although inevitably they will). The undocumented parameter is bad, but there is a clear indication that we want to remove it, especially in the comments that were removed in this patch.

The justification for this "fix" is "well, it's a terrible design, but lets keep it that way --- we will remove the intent to fix this correctly, and replace one private API with another one".

I don't care a whole lot about replacing one private API with another one, but comments should absolutely not be removed.

I don't think any of your code that you're waiting to check in is actually blocked on this, so it doesn't really make sense. PS I'm not trying to sound cranky or mean or anything, but this is a non-solution, it's just changing the way the problem is written. It doesn't even shrink the codebase :(