doc: remove PG version mention in EXPLAIN output

Reported-by: Daniel Westermann

Discussion: https://postgr.es/m/GV0P278MB0419DF1A8673E8D17A6287FAD2FA9@GV0P278MB0419.CHEP278.PROD.OUTLOOK.COM

Backpatch-through: master

Author: bmomjian

doc/src/sgml/planstats.sgml

@@ -30,8 +30,6 @@
   <para>
    The examples shown below use tables in the <productname>PostgreSQL</productname>
    regression test database.
-   The outputs shown are taken from version 8.3.
-   The behavior of earlier (or later) versions might vary.
    Note also that since <command>ANALYZE</command> uses random sampling
    while producing statistics, the results will change slightly after
    any new <command>ANALYZE</command>.