perf tests: Fix Tool PMU test segfault

perf tests: Fix Tool PMU test segfault

tool_pmu__event_to_str() now handles skipped events by returning NULL,
so it's wrong to re-check for a skip on the resulting string. Calling
tool_pmu__skip_event() with a NULL string results in a segfault so
remove the unnecessary skip to fix it:

  $ perf test -vv "parsing with PMU name"

  12.2: Parsing with PMU name:
  ...
  ---- unexpected signal (11) ----
  12.2: Parsing with PMU name         : FAILED!

Fixes: ee8aef2d2321 ("perf tools: Add skip check in tool_pmu__event_to_str()")
Signed-off-by: James Clark <james.clark@linaro.org>
Reported-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Acked-by: Kan Liang <kan.liang@linux.intel.com>
Tested-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250212163859.1489916-1-james.clark@linaro.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>