Drop redundant fields from VCS::FileInBranch

[FinnWoelm]

VCS::FileInBranch replicates some fields from current_version:

• parent_id
• name
• mime_type
• content_version
• is_deleted
• thumbnail_id

These fields should be removed and the attributes be taken from current_version. This will avoid potential data inconsistency.

[FinnWoelm]

For dropping parent_id, we need to check its consequences. The parent_id here is a reference to a VCS::FileInBranch. It is not the same as the parent_id in current_version which refers to a VCS::File.