Remove unnecessary "throws" declarations

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=201058582

javatests/google/registry/reporting/billing/BillingEmailUtilsTest.java

@@ -128,7 +128,7 @@ public class BillingEmailUtilsTest {
           private int count = 0;
 
           @Override
-          public Message answer(InvocationOnMock invocation) throws Throwable {
+          public Message answer(InvocationOnMock invocation) {
             // Once we've failed the retry limit for the original invoice, return a normal message
             // so we can properly check its contents.
             if (count < RETRY_COUNT) {