perf tools: Use __maybe_unused consistently

Instead of defining __unused or redefining __maybe_unused.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-4eleto5pih31jw1q4dypm9pf@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/jvmti/jvmti_agent.h

@@ -5,8 +5,6 @@
 #include <stdint.h>
 #include <jvmti.h>
 
-#define __unused __attribute__((unused))
-
 #if defined(__cplusplus)
 extern "C" {
 #endif