Remove old GUC name mapping for "force_parallel_mode"

This GUC was renamed to debug_parallel_query in 5352ca22e.  That commit
added an entry into map_old_guc_names[] to allow the old name still to
work.  That was done to allow a transition time where the buildfarm
configs could be swapped over to use debug_parallel_query instead.  That
work is now complete.

Here we remove the old name with the intention of breaking any user code
which is using force_parallel_query.  As mentioned in the commit message
for 5352ca22e, it appeared many users were misled into thinking that
setting this GUC was doing something useful for them to make queries run
more quickly.

Discussion: https://postgr.es/m/CAApHDvoR7EOz7Tvyzrd18FO-Dw2Cp4Uyq25TEWguK+XyCJtzOw@mail.gmail.com

src/backend/utils/misc/guc.c

@@ -186,7 +186,6 @@ static const unit_conversion time_unit_conversion_table[] =
 static const char *const map_old_guc_names[] = {
 	"sort_mem", "work_mem",
 	"vacuum_mem", "maintenance_work_mem",
-	"force_parallel_mode", "debug_parallel_query",
 	NULL
 };