coredump: Stop setting signal->group_exit_task

Currently the coredump code sets group_exit_task so that
signal_group_exit() will return true during a coredump.  Now that the
coredump code always sets SIGNAL_GROUP_EXIT there is no longer a need
to set signal->group_exit_task.

Link: https://lkml.kernel.org/r/20211213225350.27481-6-ebiederm@xmission.com


Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>