chore(Scope): remove deregisterNotifier feature for $watch

We no longer have a need for this feature that was added to primarily support
$watchGroup (see previous commit).

BREAKING CHANGE: deregisterNotifier callback for $watch is no longer available

This api was available only in the last few 1.3 beta versions and is not
very useful for applications, so we don't expect that anyone will be affected
by this change.