FROMLIST: binder: remove dead code in binder_get_ref_for_node

(from https://patchwork.kernel.org/patch/9817819/

)

node is always non-NULL in binder_get_ref_for_node so the
conditional and else clause are not needed

Change-Id: I23f011ba59e1869d9577e6bf28e1f1dd38f45713
Signed-off-by: Todd Kjos <tkjos@google.com>