Remove duplicated "Can be None". The allowReturnNones = true generates "May
return `None`.".

Fixes https://github.com/bazelbuild/bazel/issues/1893

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MOS_MIGRATED_REVID=135419555

src/main/java/com/google/devtools/build/lib/actions/Artifact.java

diff --git a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
index ac2cebf..b7ac0df 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
@@ -319,7 +319,7 @@
   }
 
   @SkylarkCallable(name = "owner", structField = true, allowReturnNones = true,
-    doc = "A label of a target that produces this File. Can be None."
+    doc = "A label of a target that produces this File."
   )
   public Label getOwnerLabel() {
     return owner.getLabel();