Simplify ExecutorRun's API and save some trivial number of cycles by having

Simplify ExecutorRun's API and save some trivial number of cycles by having
it just return void instead of sometimes returning a TupleTableSlot.  SQL
functions don't need that anymore, and noplace else does either.  Eliminating
the return value also means one less hassle for the ExecutorRun hook functions
that will be supported beginning in 8.4.