Remove obsolete freeze exception

We recently switched to a snapshot of the "debian" APT repository that has the packages we want (packages built from src:firmware-nonfree 20210818-1) in sid, i.e. where we usually pull them from.

So let's remove this freeze exception, which will allow us to proceed with #18558 (closed).

This does not impact the contents of the USB/ISO images: the resulting .packages file is identical to what we have on stable, so I don't think it's worth waiting for CI results.

This reverts commit bcd1f732.

refs #18558 (closed)