[Protocol3] allow offchainData even when onchainDataAvailability is on (#290)

* allow offchainData when onchainDataAvailability is on

* Update IExchange.sol

Author: kongliangzhong

packages/loopring_v3/contracts/iface/IExchange.sol

@@ -503,7 +503,7 @@ contract IExchange
     ///            - Mode 0: No compression. The data following the mode byte is used as is.
     ///            - Mode 1: An IDecompressor address (20 bytes) is stored after the mode byte.
     ///                      IDecompressor.decompress() will be called to decompress the following data.
-    /// @param offchainData Arbitrary data for off-chain data-availability, i.e.,
+    /// @param offchainData Arbitrary data, mainly for off-chain data-availability, i.e.,
     ///        the multihash of the IPFS file that contains the block data.
     function commitBlock(
         uint8  blockType,

packages/loopring_v3/contracts/impl/libexchange/ExchangeBlocks.sol

@@ -69,10 +69,6 @@ library ExchangeBlocks
         )
         internal  // inline call
     {
-        if (S.onchainDataAvailability) {
-            require(offchainData.length == 0, "INVALID_OFFCHAIN_DATA");
-        }
-
         commitBlockInternal(
             S,
             ExchangeData.BlockType(blockType),