Remove unused, obsolete check

From autoconf info Obsolete Macros:

"These days, it is portable to assume C89, and that signal
handlers return void, without needing to use this macro or RETSIGTYPE."

And we indeed assume so.

Signed-off-by: Ruben Kerkhof <ruben@rubenkerkhof.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>