perf stat: Clear sample_(type|period) for counting

Clear sample_(type|period) for counting, as it only confuses debug
output with unwanted sampling details:

Before:

  $ sudo perf stat -e 'raw_syscalls:sys_enter' -vv ls
  ------------------------------------------------------------
  perf_event_attr:
    type                             2
    size                             112
    config                           0x11
    { sample_period, sample_freq }   1
    sample_type                      TIME|CPU|PERIOD|RAW
    read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
    disabled                         1
    inherit                          1
    enable_on_exec                   1
    exclude_guest                    1
  ...

After:
  $ sudo perf stat -e 'raw_syscalls:sys_enter' -vv ls
  ------------------------------------------------------------
  perf_event_attr:
    type                             2
    size                             112
    config                           0x11
    read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
    disabled                         1
    inherit                          1
    enable_on_exec                   1
    exclude_guest                    1
  ...

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1448465815-27404-1-git-send-email-jolsa@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>