All callers are for MAP__FUNCTION, so just ditch it and use
thread__find_symbol(), that already ditched MAP__FUNCTION, i.e.
internally uses it till we ditch it for good.

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: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-i0ocxs00b4a0tlrx31lyh2cs@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>