Remove Tuplesortstate.copytup function

It's currently unclear how do we split functionality between
Tuplesortstate.copytup() function and tuplesort_put*() functions.
For instance, copytup_index() and copytup_datum() return error while
tuplesort_putindextuplevalues() and tuplesort_putdatum() do their work.
This commit removes Tuplesortstate.copytup() altogether, putting the
corresponding code into tuplesort_put*().

Discussion: https://postgr.es/m/CAPpHfdvjix0Ahx-H3Jp1M2R%2B_74P-zKnGGygx4OWr%3DbUQ8BNdw%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Pavel Borisov, Maxim Orlov, Matthias van de Meent
Reviewed-by: Andres Freund, John Naylor