Revert "logging.h: undefine DEBUG, ERROR, and WARN"

This reverts commit db216729ced1b2cf72b716325271c7207347cd39.

We don't need this as the problem is with the double expansion of the
LOG() macros, rather than the LOGARGS() macros. The LOG() macros invoke
LOGARGS expanding the parameter before it is passed to the second macro.

Change-Id: Iaa2a28321ec0330ecbc90eb22b51ac6e898c3329
Reviewed-on: http://review.couchbase.org/38079
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>