perf annotate: Do not overwrite perf_sample->weight

perf annotate: Do not overwrite perf_sample->weight

When we parse an event we may get a value from the kernel in response to
PERF_SAMPLE_WEIGHT being set in perf_event_attr->sample_type, and if it
is not set, then perf_sample->weight will be set to zero, which should
be ok according to a discussion with Andi Kleen [1]:

1: https://lkml.kernel.org/r/20170724174637.GS3044@two.firstfloor.org

Acked-by: Andi Kleen <andi@firstfloor.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Taeung Song <treeze.taeung@gmail.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-8ev8ufk3lzmvgz37yg9nv3qz@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>