firewall-util: drop FirewallContext

After iptables support is dropped, FirewallContext is a trivial
wrapper of sd_netlink. Let's drop it and directly use sd_netlink.

src/basic/forward.h

@@ -284,7 +284,6 @@ typedef struct ConfigTableItem ConfigTableItem;
 typedef struct CPUSet CPUSet;
 typedef struct FDSet FDSet;
 typedef struct Fido2HmacSalt Fido2HmacSalt;
-typedef struct FirewallContext FirewallContext;
 typedef struct GroupRecord GroupRecord;
 typedef struct Image Image;
 typedef struct ImagePolicy ImagePolicy;