perf tools: Remove unused PYRF_OBJS variable on Makefile

It seems that the PYRF_OBJS variable is not used anymore or has no
effect at least.  The util/setup.py tracks its dependency using
util/python-ext-sources file and resulting objects are saved under
python_ext_build/tmp/.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1348824728-14025-3-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>