Remove obsolete comment in pg_stat_statements.
Commit
76db9cb63 removed the use of multiple nesting counters,
but missed one comment describing that arrangement.
Back-patch to v17 where
76db9cb63 came in, just to avoid confusion.
Julien Rouhaud
Discussion: https://postgr.es/m/gfcwh3zjxc2vygltapgo7g6yacdor5s4ynr234b6v2ohhuvt7m@gr42joxalenw