filemanager: remove object table

[mmalenic]

The object table doesn't serve much of a purpose anymore and should be removed, as we decided to use the attributes column on s3_object to store any attributes.

If there is a requirement to have attributes follow relational modelling more closely (instead of JSON), or to store non-S3 data, then another table can be added back in. This would probably look slightly different to the current objects table anyway as it wouldn't be a 1-many relationship.

[mmalenic]

Related to #376.