refactor(ngClass): remove redundant `$observe`r and dependency on `$animate`

Includes the following commits (see #15246 for details):

- **refactor(ngClass): remove unnecessary dependency on `$animate`**

- **refactor(ngClass): remove redundant `$observe`r**

  The code was added in b41fe9f in order to support having both `ngClass` and
  interpolation in `class` work together. `ngClass` has changed considerably since
  b41fe9f and for simple cases to work the `$observe`r is no longer necessary (as
  indicated by the expanded test still passing).

  That said, it is a [documented known issue][1] that `ngClass` should not be used
  together with interpolation in `class` and more complicated cases do not work
  anyway.

[1]: https://docs.angularjs.org/api/ng/directive/ngClass#known-issues