Fix sender email address for invoicing alerts

It was failing to send alert emails because the email address it was
constructing did not have permission through GAE to send emails. This switches
it over to using the send from email address already in use elsewhere in the app
that does successfully send emails.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=219812019

java/google/registry/config/RegistryConfigSettings.java

@@ -161,7 +161,6 @@ public class RegistryConfigSettings {
     public String sheetExportId;
     public String alertRecipientEmailAddress;
     public String spec11ReplyToEmailAddress;
-    public String alertEmailSenderDomain;
     public int asyncDeleteDelaySeconds;
   }