Removed redundant assignement.

Fix 6ce8991 introduces setup of destination_key in CMJob ctor, so
re-assignement of destination_key was redundant.
Fix of HEX-842: deleting cached CMs.

The class water.api.RF forgets to clean the last CM (full forest CM)
causing showing the improper cached results if the same model key was
used in RF call.

Furthermore, RFScore now always deletes CMs.
Merge branch 'master' of github.com:0xdata/h2o