Allow the beam parameter in RDE standard mode (#1505)

Standard mode will determine the watermarks based on the cursors and
kick off subsequent uploading steps. In order to run both the Beam and
the Mapreduce pipeline in parallel, we need to allow setting the beam
parameter when in standard mode. This changes should have been part of
https://github.com/google/nomulus/pull/1500.

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core/src/main/java/google/registry/rde/RdeStagingAction.java

@@ -391,9 +391,6 @@ public final class RdeStagingAction implements Runnable {
     if (revision.isPresent()) {
       throw new BadRequestException("Revision parameter not allowed in standard operation");
     }
-    if (beam) {
-      throw new BadRequestException("Beam parameter not allowed in standard operation");
-    }
 
     return ImmutableSetMultimap.copyOf(
         Multimaps.filterValues(