Remove reference to default wal_buffers being 8

This hasn't been true since 9.1, when the default was changed to -1.
Remove the reference completely, keeping the discussion of the parameter
and it's shared memory effects on the config page.

doc/src/sgml/wal.sgml

@@ -540,9 +540,7 @@
    from having to do writes.  On such systems
    one should increase the number of <acronym>WAL</acronym> buffers by
    modifying the configuration parameter <xref
-   linkend="guc-wal-buffers">.  The default number of <acronym>WAL</acronym>
-   buffers is 8.  Increasing this value will
-   correspondingly increase shared memory usage.  When
+   linkend="guc-wal-buffers">.  When
    <xref linkend="guc-full-page-writes"> is set and the system is very busy,
    setting this value higher will help smooth response times during the
    period immediately following each checkpoint.