dm cache metadata: remove unused struct 'thunk'

'thunk' has been unused since commit f177940a ("dm cache metadata:
switch to using the new cursor api for loading metadata").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>