Adapt to JGit 5.3.x for Gerrit v3.0

Gerrit v3.0 includes JGit 5.3.x which does not have the
MultiSiteRefDatabase.getRef() method anymore.
Hence it needs to be removed from the multi-site wrapper
also.

Change-Id: Ie00f451d737b06054f4295449b1bb2867c1eecff
diff --git a/src/main/java/com/googlesource/gerrit/plugins/multisite/validation/MultiSiteRefDatabase.java b/src/main/java/com/googlesource/gerrit/plugins/multisite/validation/MultiSiteRefDatabase.java
index 630b091..e1d1c65 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/multisite/validation/MultiSiteRefDatabase.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/multisite/validation/MultiSiteRefDatabase.java
@@ -99,11 +99,6 @@
   }
 
   @Override
-  public Ref getRef(String name) throws IOException {
-    return refDatabase.getRef(name);
-  }
-
-  @Override
   public String toString() {
     return refDatabase.toString();
   }