qcow2: Remove unused fields from BDRVQcow2State

qcow2: Remove unused fields from BDRVQcow2State

These fields were already removed in commit c3c10f72, but then commit
b58deb34 revived them probably due to bad merge conflict resolution.
They are still unused, so remove them again.

Fixes: b58deb344ddff3b9d8b265bf73a65274767ee5f4
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200326170757.12344-1-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>