Real-time collaboration: disable for site editor.

In performance tests, editing large documents in the site-editor suite (loading large-post.html) can cause performance tests in the site editor to time out with memory errors.

Developed in: https://github.com/WordPress/wordpress-develop/pull/11181.

Props alecgeatches, mukesh27.
See #64622.

git-svn-id: https://develop.svn.wordpress.org/trunk@61986 602fd350-edb4-49c9-b593-d223f7449a82