Remove extensions not yet compatible with GNOME Shell 47

[Jeremy Bícha]

Please remove gnome-shell-pomodoro from oracular. It is not yet compatible with GNOME Shell 47. A simple update of the metadata.json file is insufficient for the extension to be able to be activated. It is likely that this will be fixed before the release of Ubuntu 24.10 but we want to land GNOME Shell 47 quickly.

Please also remove gnome-shell-mailnag from oracular. Testing showed that the extension doesn't work well in GNOME Shell 46. Although it could be updated easily to be installable with GNOME Shell 47, I believe it would be better for our users for this extension to be removed. Optionally, it could be added to the sync blocklist like most GNOME Shell extensions. An exception was made previously because this extension depends on a packaged system service (mailnag).