perf tests: Stop Convert perf time to TSC test opening events twice

Do not call evlist__open() twice.

Fixes: 5bb017d4

 ("perf test: Fix error message for test case 71 on s390, where it is not supported")
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/20220713123459.24145-2-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>