At the time replacement_sort_tuples was introduced, there were still
cases where replacement selection sort noticeably outperformed using
quicksort even for the first run.  However, those cases seem to have
evaporated as a result of further improvements made since that time
(and perhaps also advances in CPU technology).  So remove replacement
selection and the controlling GUC entirely.  This makes tuplesort.c
noticeably simpler and probably paves the way for further
optimizations someone might want to do later.

Peter Geoghegan, with review and testing by Tomas Vondra and me.

Discussion: https://postgr.es/m/CAH2-WzmmNjG_K0R9nqYwMq3zjyJJK+hCbiZYNGhAy-Zyjs64GQ@mail.gmail.com