ptrace: Collapse ptrace_untrace() into __ptrace_unlink()

Remove the extra task_is_traced() check in __ptrace_unlink() and
collapse ptrace_untrace() into __ptrace_unlink().  This is to prepare
for further changes.

While at it, drop the comment on top of ptrace_untrace() and convert
__ptrace_unlink() comment to docbook format.  Detailed comment will be
added by the next patch.

This patch doesn't cause any visible behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>