BibDocFile: revert md5 property patch

• Reverts md5 property patch 18c013cd7e511cb83d742e70bb0972e426ae0b0e that was defining only getter, not setter. This caused problems during recreation of demo site. (references #1249)

• Cherry-picked from 13ba6527d50f0cd8dae0763e0268ae79f5e5fcf6 that reverted this patch on master branch only.

Reported-by: Patrick Glauner <patrick.oliver.glauner@cern.ch>