Remove obsolete @todo for "Undo bug when first inserting media into unfocused CKEditor"

Problem/Motivation

This is a follow up to #2994699: Create a CKEditor plugin to select and embed a media item from the Media Library, and is blocked on that issue.

In #2994699-49: Wim Leers gave steps to reproduce a bug with the DrupalMediaLibrary CKEditor button's interaction with the "undo" button:

Steps to reproduce:

1. Go to /node/add/article
2. Click the "Insert from Media Library" button without giving focus to anything in CKEditor first
3. Select media, insert it

Expected: Undo button is active.
Actual: It is not active.

If before step 2 you click inside the CKEditor iframe instance, or for example type something first, then the undo button does work as expected.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet