AKI-166: Fix to kernelInterface removeStorage

- The removeStorage() method was incorrectly doing an addStorage and
  overwriting the value with byte[]{0}. This has been fixed to properly
  delete the storage value. The avm relies on there being a difference
  between null and non-null, so it would be incorrect to treat missing/null
  as a non-null entry from its perspective. This code path affects the
  getStorage and putStorage behaviour that the Avm exposes.

modVM/src/org/aion/vm/KernelInterfaceForAVM.java

@@ -137,7 +137,7 @@ public void putStorage(Address address, byte[] key, byte[] value) {
     @Override
     public void removeStorage(Address address, byte[] key) {
         ByteArrayWrapper storageKey = new ByteArrayWrapper(key);
-        this.repositoryCache.addStorageRow(address, storageKey, ByteArrayWrapper.ZERO);
+        this.repositoryCache.removeStorageRow(address, storageKey);
         setVmType(address);
     }