packaging: Drop unnecessary cockpit-system dependency

All cockpit extensions should only require the bridge, at least as long
as they are independent from any other page (which is usually the case).
We still need cockpit-system for the tests (thus keep it in
test/browser/main.fmf), but it is perfectly reasonable to look at a
particular Cockpit page via the "remote hosts" panel, cockpit-desktop,
or Cockpit Client.

packaging/cockpit-starter-kit.spec.in

@@ -19,7 +19,7 @@ BuildRequires: gettext
 BuildRequires: libappstream-glib-devel
 %endif
 
-Requires: cockpit-system
+Requires: cockpit-bridge
 
 %description
 Cockpit Starter Kit Example Module