perf tools: Remove unused 'prefix' from builtin functions

We got it from the git sources but never used it for anything, with the
place where this would be somehow used remaining:

  static int run_builtin(struct cmd_struct *p, int argc, const char **argv)
  {
	prefix = NULL;
	if (p->option & RUN_SETUP)
		prefix = NULL; /* setup_perf_directory(); */

Ditch it.

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-uw5swz05vol0qpr32c5lpvus@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/builtin-timechart.c b/tools/perf/builtin-timechart.c
index fbd7c6c..fafdb44 100644
--- a/tools/perf/builtin-timechart.c
+++ b/tools/perf/builtin-timechart.c
@@ -1773,7 +1773,7 @@ static int timechart__io_record(int argc, const char **argv)
 	for (i = 0; i < (unsigned int)argc; i++)
 		*p++ = argv[i];
 
-	return cmd_record(rec_argc, rec_argv, NULL);
+	return cmd_record(rec_argc, rec_argv);
 }
 
 
@@ -1864,7 +1864,7 @@ static int timechart__record(struct timechart *tchart, int argc, const char **ar
 	for (j = 0; j < (unsigned int)argc; j++)
 		*p++ = argv[j];
 
-	return cmd_record(rec_argc, rec_argv, NULL);
+	return cmd_record(rec_argc, rec_argv);
 }
 
 static int
@@ -1917,8 +1917,7 @@ parse_time(const struct option *opt, const char *arg, int __maybe_unused unset)
 	return 0;
 }
 
-int cmd_timechart(int argc, const char **argv,
-		  const char *prefix __maybe_unused)
+int cmd_timechart(int argc, const char **argv)
 {
 	struct timechart tchart = {
 		.tool = {