Planner: after freeing the plan, throw away the pointer

Otherwise we'll happily go back and access the freed data structures
later. Oops.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: remove pointless code

This never made any sense. If we have NO datapoints then go and cancel the
plan? And anyway, we already check this higher up in the call stack.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>