Drive-by: remove reference to BrOnExnNull in wasm-module-builder.js.

R=clemensb@chromium.org
CC=aheejin@chromium.org

Bug: v8:8091
Change-Id: I42821b21c32fe8bf3410e75cf81bbff9678d3fa9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2575059
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71766}