Finish refactoring make_foo() functions in createplan.c.

Finish refactoring make_foo() functions in createplan.c.

This patch removes some redundant cost calculations that I left for later
cleanup in commit 3fc6e2d7f5b652b4.  There's now a uniform policy that the
make_foo() convenience functions don't do any cost calculations.  Most of
their callers copy costs from the source Path node, and for those that
don't, the calculation in the make_foo() function wasn't necessarily right
anyhow.  (make_result() was particularly a mess, as it was serving multiple
callers using cost calcs designed for only the first one or two that had
ever existed.)  Aside from saving a few cycles, this ensures that what
EXPLAIN prints matches the costs we used for planning purposes.  It does
not change any planner decisions, since the decisions are already made.