fix($animate): avoid memory leak with `$animate.enabled(element, enabled)`

When disabling/enabling animations on a specific element (via
`$animate.enabled(element, enabled)`), the element is added in a map to
track its state. Previously, the element was never removed from the map,
causing AngularJS to hold on to the element even after it is removed
from the DOM, thus preventing it from being garbage collected.

This commit fixes it by removing the element from the map on `$destroy`.

Fixes #16637.

Closes #16649