clients: ivi-shell-user-interface, Delete signal handlings

> I don't think you need to catch these at all. The default action is to
> kill the process, which is quite fine. The default action even sets the
> process exit code properly, unlike these.

Signed-off-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>