fix(data): make undoMany remove tracking changes in changeState (#2346)

[AdditionAddict]

Fixes #2346

PR Checklist

Please check if your PR fulfills the following requirements:

• [ x ] The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• [ x ] Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ x ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

UNDO_ONE and UNDO_MANY actions reverts one/many entities back to their original state but changeState keeps the tracking the entity/entities.

Closes #2346

What is the new behavior?

UNDO_ONE and UNDO_MANY actions remove the entity/entities from changeState as well as the current behavior of reverting the entities state.

Does this PR introduce a breaking change?

[ ] Yes
[ x ] No

Other information

This would be my first ever PR if accepted.

Please confirm if PR is intended behavior before accepting.

Reference https://ngrx.io/guide/data/entity-change-tracker#removing-an-entity-from-the-changestate-map

The undo operations replace entities in the collection based on information in the changeState map, reverting them their last known server-side state, and removing them from the changeState map. These entities become "unchanged."

[AdditionAddict]

Does Documentation refer to answering the PR questions or the actual website documentation?

[ngrxbot]

Preview docs changes for 04658a5 at https://previews.ngrx.io/pr2352-04658a5/

[brandonroberts]

@AdditionAddict it refers to website documentation. From the excerpt, it seems that this fixes the behavior described there.