kernel: Remove signal hacks for vhost_tasks

This removes the signal/coredump hacks added for vhost_tasks in:

Commit f9010dbd

 ("fork, vhost: Use CLONE_THREAD to fix freezer/ps regression")

When that patch was added vhost_tasks did not handle SIGKILL and would
try to ignore/clear the signal and continue on until the device's close
function was called. In the previous patches vhost_tasks and the vhost
drivers were converted to support SIGKILL by cleaning themselves up and
exiting. The hacks are no longer needed so this removes them.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20240316004707.45557-10-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>