Remove vestigial serialization bits from ErrorInfo

RELNOTES:

--
MOS_MIGRATED_REVID=104674937

src/main/java/com/google/devtools/build/skyframe/ErrorInfo.java

diff --git a/src/main/java/com/google/devtools/build/skyframe/ErrorInfo.java b/src/main/java/com/google/devtools/build/skyframe/ErrorInfo.java
index 72e7ab4..550466d 100644
--- a/src/main/java/com/google/devtools/build/skyframe/ErrorInfo.java
+++ b/src/main/java/com/google/devtools/build/skyframe/ErrorInfo.java
@@ -21,7 +21,6 @@
 import com.google.devtools.build.lib.collect.nestedset.Order;
 import com.google.devtools.build.skyframe.SkyFunctionException.ReifiedSkyFunctionException;
 
-import java.io.IOException;
 import java.util.Collection;
 
 import javax.annotation.Nullable;
@@ -89,8 +88,7 @@
         isCatastrophic);
   }
 
-  // Non-final only to allow for serialization.
-  private transient NestedSet<SkyKey> rootCauses;
+  private final NestedSet<SkyKey> rootCauses;
 
   @Nullable private final Exception exception;
   private final SkyKey rootCauseOfException;
@@ -179,8 +177,4 @@
     return isCatastrophic;
   }
 
-  private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException {
-    in.defaultReadObject();
-    rootCauses = NestedSetBuilder.emptySet(Order.STABLE_ORDER);
-  }
 }