The global log buffer feature was deleted in:

  commit c0c8c1d7
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Mon Mar 3 14:54:33 2014 +0000

    Remove global log buffer feature entirely

    A earlier commit changed the global log buffer so that it only
    records messages that are explicitly requested via the log
    filters setting. This removes the performance burden, and
    improves the signal/noise ratio for messages in the global
    buffer. At the same time though, it is somewhat pointless, since
    all the recorded log messages are already going to be sent to an
    explicit log output like syslog, stderr or the journal. The
    global log buffer is thus just duplicating this data on stderr
    upon crash.

    The log_buffer_size config parameter is left in the augeas
    lens to prevent breakage for users on upgrade. It is however
    completely ignored hereafter.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

This was in the 1.2.3 release, and 4 years is sufficient time for a
graceful upgrade path for augeas, so all remaining traces are now
removed.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>