Revise squashfuse/squashfs-tools patches

[TheAssassin]

@probonopd mentioned that the patches we have in the repository and apply to squashfuse/squashfs-tools might no longer be necessary. They should be revised, perhaps we might no longer require them. That'd be great news, as they're more of a burden and removing them will remove some maintenance overhead.

While we're at it, we should check whether we can update either dependency. This might be especially interesting because of the new zstd compression method squashfs implemented and which we're interested in trying out.

[probonopd]

Regarding zstd, we should understand first how this plays together with zsync(2) special handling of compressed files and whether there is really a significant advantage over what we have now (some initial tests didn't show dramatic improvements in AppImage sizes, but I didn't do thorough tests yet).

So I'd say, this ticket can be done independently of our ongoing evaluation/consideration of Zstandard (which is not decided yet).

[TheAssassin]

Subtask of #815.