Removing the TRACE_SORT macro

Currently, debug instrumentation for sorting is only available if the
TRACE_SORT macro was defined when PostgreSQL was compiled. It is
defined by default, and so in practice it's always available; there is
really no upside to disabling it. "18.17. Developer Options" notes
this restriction for trace_sort, which is the only such restriction.

The number of TRACE_SORT elog() logging callers has grown
significantly in the past couple of releases. The associated "#ifdef
TRACE_SORT" crud has also grown.

I propose that we completely remove the TRACE_SORT macro, and all the
associated crud. Just having that as an option suggests that there is
some possible upside to disabling trace_sort, which is clearly not
true. I will write a patch doing this if there are no objections. I
think this is justifiable as clean-up for 9.6.

-- 
Peter Geoghegan