perf tests: No need to set attr.sample_freq for tracking !PERF_RECORD_SAMPLE

We were asking for a 4kHz sample_freq, making the test fail needlessly
when the system reduced /proc/sys/kernel/perf_event_max_sample_rate
below that.

Before:

  # perf test -vv dummy
  23: Test using a dummy software event to keep tracking       :
  --- start ---
  test child forked, pid 32421
  ------------------------------------------------------------
  perf_event_attr:
    type                             1
    size                             112
    config                           0x9
    { sample_period, sample_freq }   4000
    sample_type                      IP|TID|ID|PERIOD
  <SNIP>
  sys_perf_event_open failed, error -22
  Unable to open dummy and cycles event
  test child finished with -2
  ---- end ----
  Test using a dummy software event to keep tracking: Skip
  #
  [root@zoo ~]# cat /proc/sys/kernel/perf_event_max_sample_rate
  1000

After:

  [root@zoo ~]# perf test dummy
  23: Test using a dummy software event to keep tracking       : Ok

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-487iquegrs2379e5n0pi0tcp@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/keep-tracking.c

@@ -55,7 +55,6 @@ int test__keep_tracking(int subtest __maybe_unused)
 		.mmap_pages	     = UINT_MAX,
 		.user_freq	     = UINT_MAX,
 		.user_interval	     = ULLONG_MAX,
-		.freq		     = 4000,
 		.target		     = {
 			.uses_mmap   = true,
 		},