This reverts commit 7e0bfad2.

A late objection to the ABI has arrived:

   http://lkml.org/lkml/2009/6/10/253

Keep the ABI disabled out of caution, to not create premature
user-space expectations.

While the hw-branch-tracing variant uses and tests the BTS code.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Markus Metzger <markus.t.metzger@intel.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>