perf thread-stack: Tidy thread_stack__bottom() usage

In preparation for fixing thread stack processing for the idle task,
tidy thread_stack__bottom() usage. Specifically, the parameter 'thread'
is not needed.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20181221120620.9659-3-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>