Drop the flow states GK_GRANTED and GK_DECLINED

Once pull request #295 is merged, the flow states `GK_GRANTED` and `GK_DECLINED` become redundant since the new flow state `GK_BPF` can emulate them. Dropping the flow states `GK_GRANTED` and `GK_DECLINED` will reduce and simplify the code. If some backward compatibility is needed at the GGU protocol, the dropped states can be easily emulated mapping the old policy decision formats onto their corresponding BPF programs.