[Libguestfs] Remove 3 unused fields from the filesystem struct

[Matthew Booth]

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...