Don't serve gwtjsonrpc endpoints if GWT UI isn't enabled
Don't serve gwtjsonrpc endpoints if GWT UI isn't enabled

The old-style RPC endpoints are not a public interface and practically
speaking can't really be used except by GWT code; there's no point in
serving them if there is no GWT UI.

Change-Id: I5abe6ca48448d6eca3f47dbaf681c1553e637203

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