Print planning time only in EXPLAIN ANALYZE, not plain EXPLAIN.

We've gotten enough push-back on that change to make it clear that it
wasn't an especially good idea to do it like that.  Revert plain EXPLAIN
to its previous behavior, but keep the extra output in EXPLAIN ANALYZE.
Per discussion.

Internally, I set this up as a separate flag ExplainState.summary that
controls printing of planning time and execution time.  For now it's
just copied from the ANALYZE option, but we could consider exposing it
to users.