perf tools: Remove needless thread.h include directives

perf tools: Remove needless thread.h include directives

Now that thread.h isn't included by any other header, we can check where
it is really needed, i.e. we can remove it and be sure that it isn't
being obtained indirectly.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-kh333ivjbw05wsggckpziu86@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>