FFI: Always fall back to metamethods for cdata length/concat.

Thanks to Egor Skriptunoff.

src/lj_carith.h

@@ -11,7 +11,6 @@
 #if LJ_HASFFI
 
 LJ_FUNC int lj_carith_op(lua_State *L, MMS mm);
-LJ_FUNC int lj_carith_len(lua_State *L);
 
 #if LJ_32 && LJ_HASJIT
 LJ_FUNC int64_t lj_carith_mul64(int64_t x, int64_t k);