Revert "vhost_task: Handle SIGKILL by flushing work and exiting"

This reverts commit abe067dc which is
commit db5247d9 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Ia5e187cd927f6cea91719b7ca9894fefc6c1eb62
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>