Allow multiple DNS writers on TLDs

This completes the data/functionality migration for multiple DNS writers.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=163835077

java/google/registry/model/registry/Registry.java

@@ -676,8 +676,6 @@ public class Registry extends ImmutableObject implements Buildable {
     }
 
     public Builder setDnsWriters(ImmutableSet<String> dnsWriters) {
-      // TODO(b/63385597): Remove this restriction once DNS task queue migration is complete.
-      checkArgument(dnsWriters.size() == 1, "Multiple DNS writers are not yet supported");
       getInstance().dnsWriters = dnsWriters;
       return this;
     }