perf test: Remove dangling CFLAGS for removed attr.o object

Since the C test wrapper for attr.py was removed we don't have an attr.o
object for that CFLAGS_attr.o to apply for, remove it.

Fixes: 3a447031 ("perf test: Remove C test wrapper for attr.py")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Leo Yan <leo.yan@linux.dev>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Veronika Molnarova <vmolnaro@redhat.com>
Cc: Weilin Wang <weilin.wang@intel.com>
Cc: Ze Gao <zegao2021@gmail.com>
Cc: zhaimingbing <zhaimingbing@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/ZyjbksKYnV22zmz-@x1


Signed-off-by: Namhyung Kim <namhyung@kernel.org>