mgmt: delete candidate scratch buffer

mgmt: delete candidate scratch buffer

The code doesn't work at all. It tries to use libyang operation
metadata in a regular (not diff) data tree, and regular data trees
don't provide this data. Also, for destroy operations, it searches
for nodes in the running config, which may not have the deleted nodes
if we're not using implicit commits.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>