Jetpack: Contact Form: Hide legacy Coblocks form from the inserter on Atomic sites

On Atomic sites there is a "second" form block available (served via Co-blocks plugin that is autoinstalled)


Obviously confusing, especially later when users might used both forms at some point and are wondering about where their data ended up.

Start by locating the Jetpack Contact Form block registration and the inserter behavior for Atomic sites, then compare it with the legacy Co-blocks form that is autoinstalled. Done means the legacy form no longer appears in the inserter on Atomic sites while the Jetpack Contact Form remains available.

Written by the indexing model from the issue text.