How to delete all but the current revision from the field_collection_item_revision table?
(Assuming a MySQL database)

Execute the following MySQL request:

DELETE fcir FROM field_collection_item_revision AS fcir
  RIGHT JOIN field_collection_item AS fci 
    ON fcir.item_id = fci.item_id
  WHERE NOT fcir.revision_id = fci.revision_id;


We delete all revisions that are not present in the field_collection_item table under "revision_id".