Remove real-time collaboration.

Removes all RTC related code from core. There will still be RTC related code in Gutenberg, but effectively disabled for core since nothing turns it on. The wp.sync global has also been hidden by bundling in https://github.com/WordPress/gutenberg/pull/78085.

Developed in: https://github.com/WordPress/wordpress-develop/pull/11774.

Props maxschmeling, ellatrix, mukesh27.
Fixes #65205.

git-svn-id: https://develop.svn.wordpress.org/trunk@62334 602fd350-edb4-49c9-b593-d223f7449a82