core: Remove GNOME Shell Extensions

This is a recognition that GNOME does not consider GNOME Classic to be part of core GNOME. ("Out of scope")

Previously GNOME OS included the Classic extensions without either the GNOME Classic session file or an extensions app to enable those extensions.

Although a few GNOME distributions do include GNOME Classic in their default install, no current major distro uses it as their default GNOME session. (RHEL 8, released in 2019, used the usual GNOME session as the default GNOME session.)

No changes should be necessary to gnome-shell-extensions because of this change.

I have previously discussed this with the GNOME Shell extensions maintainer, @fmuellner, who agreed with this removal.

I believe this change will not be disruptive to distros at all and believe that this change can be part of GNOME 48 as long as we merge this before GNOME 48 Alpha.

There is a procedure for app removal but Florian and I agree that GNOME Classic is not really an app and this removal does not significantly conflict with those procedures anyway.