d8 treats files with the .mjs extension as modules instead of
classic scripts. Thus, the `// MODULE` pragma and its corresponding
logic in test runners can be removed in favor of explicitly adding
the extension.

Bug: v8:7950, v8:9395, v8:9406
Also-By: tmrts@chromium.org
Change-Id: Ic74328dc5c5f176bb4bdf6d74bdd4d3966279ba5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1675958
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Auto-Submit: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62421}