perf session: Remove wrappers to machines__find

perf session: Remove wrappers to machines__find

Start to untangle session from delivering samples, as there are
tools that want to use ordered_events and don't use perf_session at all.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-rn4pk3pjxd78sgzrkn19tktp@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>