Remove @OverRide from blobExists

As of this commit[1] `blobExists` method has been removed from the
`BlobContainer` class.

[1]: elastic/elasticsearch@4b8fd4e

src/main/java/org/wikimedia/elasticsearch/swift/repositories/blobstore/SwiftBlobContainer.java

@@ -71,7 +71,6 @@ protected SwiftBlobContainer(BlobPath path, SwiftBlobStore blobStore) {
     /**
      * Does a blob exist? Self-explanatory.
      */
-    @Override
     public boolean blobExists(final String blobName) {
         return SwiftPerms.exec(() -> blobStore.swift().getObject(buildKey(blobName)).exists());
     }