firewall-util: remove iptables/libiptc backend support (#38976)

This removes iptables/libiptc backend support in firewall-util, as
already announced by 5c68c51045.
Then, this drops meaningless `FirewallContext` wrapper.

src/basic/build.c

@@ -126,12 +126,6 @@ const char* const systemd_features =
         " -IDN"
 #endif
 
-#if HAVE_LIBIPTC
-        " +IPTC"
-#else
-        " -IPTC"
-#endif
-
 #if HAVE_KMOD
         " +KMOD"
 #else

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;