x86, ds: dont use TIF_DEBUGCTLMSR

Debug store already uses TIF_DS_AREA_MSR to trigger debug store context
switch handling. No need to use TIF_DEBUGCTLMSR, as well.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Cc: roland@redhat.com
Cc: eranian@googlemail.com
Cc: oleg@redhat.com
Cc: juan.villacis@intel.com
Cc: ak@linux.jf.intel.com
LKML-Reference: <20090403144604.256645000@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>