Move transitional packages libiptc0, libiptc-dev to oldlibs/optional per policy 4.0.1.

Fixes lintian: transitional-package-should-be-oldlibs-optional
See https://lintian.debian.org/tags/transitional-package-should-be-oldlibs-optional.html for more details.

debian/control

@@ -84,7 +84,6 @@ Description: netfilter xtables library -- development files
 
 Package: libiptc0
 Architecture: linux-any
-Priority: optional
 Multi-Arch: same
 Depends: libip4tc2 (=${binary:Version}),
          ${misc:Depends}
@@ -94,7 +93,6 @@ Description: transitional dummy package
 
 Package: libiptc-dev
 Architecture: linux-any
-Priority: optional
 Multi-Arch: same
 Depends: libip4tc-dev (=${binary:Version}),
          ${misc:Depends}