Drop special accounting for snapshot space usage in VG.
Drop special accounting for snapshot space usage in VG.

Snapshots are now represented by devices, so the space accounting is
handled the same as for other LVs.

rvykydal/blivet,jkonecny12/blivet,jkonecny12/blivet,dwlehman/blivet,rvykydal/blivet,vojtechtrefny/blivet,rhinstaller/blivet,AdamWill/blivet,rhinstaller/blivet,vojtechtrefny/blivet,vpodzime/blivet,dwlehman/blivet,vpodzime/blivet,AdamWill/blivet