Change usage grouping key in the invoice CSV (#2024)

This column is used by the billing team to create invoices. Registrars
have asked that a single invoice be created for a given registrar,
instead of one per registrar-tld pair. This should have no other effect
on the billing pipeline as the invoice grouping key has a description
field that also contains the TLD, so the granularity as a whole does not
change.

core/src/main/java/google/registry/beam/billing/BillingEvent.java

@@ -189,7 +189,7 @@ public abstract class BillingEvent implements Serializable {
                 .minusDays(1)
                 .toString(),
         billingId(),
-        String.format("%s - %s", registrarId(), tld()),
+        registrarId(),
         String.format("%s | TLD: %s | TERM: %d-year", action(), tld(), years()),
         amount(),
         currency(),
@@ -233,7 +233,7 @@ public abstract class BillingEvent implements Serializable {
     /** Returns the billing account id, which is the {@code BillingEvent.billingId}. */
     abstract String productAccountKey();
 
-    /** Returns the invoice grouping key, which is in the format "registrarId - tld". */
+    /** Returns the invoice grouping key, which is the registrar ID. */
     abstract String usageGroupingKey();
 
     /** Returns a description of the item, formatted as "action | TLD: tld | TERM: n-year." */