Preserve plugin layers as of QGreenland v3, but discontinue support for updates

[MattF-NSIDC]

QGreenland code currently does not clean up completed layers after packaging because we want to make them available for plugin users to access dynamically. We don't want to break that workflow, but we also don't want to continue to support the plugin.

We should do something like copying the plugin layers as-is to another location, updating our webserver config to host the layers directory from that location, and update the QGreenland code to clean up after itself again.