info: Remove unused gsd-disk-space-helper.[h|c]
!387 (merged) ported info panel to use UDisks2 instead of GUnixMounts and thus the helpers from gsd-disk-space-helper.[h|c] are no more needed.
!387 (merged) ported info panel to use UDisks2 instead of GUnixMounts and thus the helpers from gsd-disk-space-helper.[h|c] are no more needed.