Remove very-obsolete estimates of shmem usage from postgresql.conf.sample.

Remove very-obsolete estimates of shmem usage from postgresql.conf.sample.

runtime.sgml used to contain a table of estimated shared memory consumption
rates for max_connections and some other GUCs.  Commit 390bfc643 removed
that on the well-founded grounds that (a) we weren't maintaining the
entries well and (b) it no longer mattered so much once we got out from
under SysV shmem limits.  But it missed that there were even-more-obsolete
versions of some of those numbers in comments in postgresql.conf.sample.
Remove those too.  Back-patch to 9.3 where the aforesaid commit went in.