Revert "[BUGFIX beta] Don't use prototype extensions (arrayContent{Did,Will}Change, removeAt)"

[rwjblue]

Reverts #13534.

After discussing @krisselden I realized that I merged #13534 prematurely (I am sorry about that). There are still a few things that need to be addressed there to allow us to drop Ember.A usage and those need to be addressed before landing again.

/cc @btecu

[rwjblue]

@krisselden was going to take a second look, and confirm before the revert...

[krisselden]

We can leave it, this doesn't expose the API as public, there is a lot that needs to be done before standalone API is made public if the end goal is to migrate off of prototype extensions, it would be a big mistake to expose the standalone methods public until they are more the equivalent to Ember.set

We need an actual plan, my guess is that we'd deprecate Ember.A without the ENV.EXTEND_PROTOTYPES.Array (you wouldn't need it for observability and you should use standalone methods for mutating), and with ENV.EXTEND_PROTOTYPES.Array on we'd deprecate it being true, but we can't do this until the standalone methods trigger a '[]'. I would like to remove the array and enumerable observer altogether.