perf(Scope): remove the need for the extra watch in $watchGroup

Instead of using a counter and an extra watch, just schedule the reaction function via $evalAsync.

This gives us the same/similar ordering and coalecsing of updates as counter without the extra
overhead. Also the code is easier to read.

Since interpolation uses watchGroup, this change additionally improves performance of interpolation.

In large table benchmark digest cost went down by 15-20% for interpolation.

Closes #8396
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.