Cause EXPLAIN's VERBOSE option to print the target list (output column list)

Cause EXPLAIN's VERBOSE option to print the target list (output column list)
of each plan node, instead of its former behavior of dumping the internal
representation of the plan tree.  The latter display is still available for
those who really want it (see debug_print_plan), but uses for it are certainly
few and and far between.  Per discussion.

This patch also removes the explain_pretty_print GUC, which is obsoleted
by the change.