libperf evsel: Factor perf_evsel__exit out of perf_evsel__delete

This allows the perf_evsel__exit to be called when the struct
perf_evsel is embedded inside another struct, such as struct evsel in
perf.

Reviewed-by: Thomas Falcon <thomas.falcon@intel.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: James Clark <james.clark@linaro.org>
Link: https://lore.kernel.org/r/20250719030517.1990983-8-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>