Remove deprecated API.

This includes TypedDatastore.

prepareAttachment, addAttachment, getAttachment,
attachmentsForRevision are still public but declared in the
DatastoreExtended interface. They need to be public as code from other
packages such as com.cloudant.sync.replication need to call them. The
JavaDoc in the interface makes this clear.

Also fixed a bug where attachments weren't getting properly cleaned up
under the new attachments API.
Fixup javadoc
Fix logic on purgeAttachments()
Fix comment
Merge pull request #42 from cloudant/35678-remove-deprecated-api

Remove deprecated API.