Remove Chat Placeholder when chat is hidden

Please ensure that the following checklist has been satisfied before submitting the PR:

[ ] The documentation has been updated, if necessary.
[ ] Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
[ ] All code follows the Scala, Java, TypeScript, and Rust style guides. In case you are using a language not listed above, follow the Rust style guide.
[ ] Unit tests have been written where possible.

enso-org / enso