powerpc: Reclaim two unused thread_info flag bits

TIF_PERFMON_WORK and TIF_PERFMON_CTXSW are completely unused.  They
appear to be related to the old perfmon2 code, which has been
superseded by the perf_event infrastructure.  This removes their
definitions so that the bits can be used for other purposes.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>