[versioning] let the file cache track the size of the versions

We no longer track the size of the versions by our own but use the file cache instead.

cc @DeepDiver1975 as discussed here https://github.com/owncloud/core/issues/8252#issuecomment-42284636

PS: Same agreement as for the removed trash bin table, you will take care of bug reports from angry people who downgraded, right  @DeepDiver1975  :wink: