perf thread: Make thread__find_map() search all maps

perf thread: Make thread__find_map() search all maps

We still have the split internally, but users don't see it anymore,
simplifying the growing number of cases where we end up searching
in the MAP__VARIABLE maps.

This further paves the way for ditching the split.

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-86mfxrztf310konutxvhr5ua@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>