Remove misleading hint for "unexpected data beyond EOF" error.

Remove misleading hint for "unexpected data beyond EOF" error.

Commit ffae5cc5a6024b4e25ec920ed5c4dfac649605f8 added this hint in 2006,
but it's now obsolete and doesn't reflect what users should really check
in this situation. We were not able to agree on a new hint, so just delete
the existing one and update the comments to mention one possibility that
is known to cause problems of this kind: something other than PostgreSQL
is modifying files in the PostgreSQL data directory.

Author: Jakub Wartak <jakub.wartak@enterprisedb.com>
Reviewed-by: Robert Haas <rhaas@postgresql.org>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/CAKZiRmxNbcaL76x=09Sxf7aUmrRQJBf8drzDdUHo+j9_eM+VMg@mail.gmail.com