[Libguestfs] Remove 3 unused fields from the filesystem struct

These 3 patches remove unused fields from the filesystem struct, and the inspection code which populates them. The second 2 are fairly trivial NFCs, but the first has the potential for side effects as it entirely removes a couple of matches from the filesystem detection code. A filesystem which was previously matched as X and ignored might conceivably now be matched as Y instead, whereas previously that test would never have happened. This may be a good thing or a bad thing...