Follow-up Id75401be (e32c8afe): no more need to prefix debug messages

Since that revision, messages sent through wfDebugLog() and without
a corresponding entry in $wgDebugLogGroups will be prefixed there
before being sent to wfDebug(), so it's no longer needed to have the
exact same checks for the debug messages in memcached clients.

Change-Id: Ia690a87bc6d1994fe371816470efde6d09e3b881
Merge "Follow-up Id75401be (e32c8afe): no more need to prefix debug messages"