Remove obsolete comment block in nbtsort.c.

Remove obsolete comment block in nbtsort.c.

Building a new nbtree index through incremental insertions would always
be slower than our actual approach of sorting using tuplesort,
assembling leaf pages from tuplesort output, and writing and WAL-logging
whole pages.  Remove a comment block from the Berkeley days claiming
that incremental insertions might be slightly faster with presorted
input.

Discussion: https://postgr.es/m/CAH2-WzmKs4mLAoFgJ3yHMRYc849efc=dw+pNRb3NEog2oJoCNw@mail.gmail.com