perf script: Use thread__find_symbol() instead of ad-hoc equivalent

perf script: Use thread__find_symbol() instead of ad-hoc equivalent

In dc323ce8e72d ("perf script: Enable printing of branch stack") it
first tries to find the map for an address, then the symbol in the DSO
backing that map, for that address, well, this is what
thread__find_symbol() does, so just use it and make the code shorter.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-03nx3aod955yqnf9l06im28j@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>