perf session: Remove superfluous callchain_cursor member

Since we have it in evsel->hists.callchain_cursor, remove it from
perf_session.

One more step in disentangling several places from requiring a
perf_session pointer.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-rxr5dj3di7ckyfmnz0naku1z@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>