build: Fix build failure after Closure upgrade (shaka-project#4063)

6f274cb dropped a deprecated Closure build tool, but also upgraded
the Closure compiler and Closure library.  This introduced a failure
caused by a duplicate extern in the newer compiler.

This drops the now-unnecessary extern.

<!--
Please remember to:

1. Use Conventional Commits syntax (fix: ..., feat: ..., etc.) in commits and
   PR title (https://www.conventionalcommits.org/)
2. Tag any related or fixed issues ("Issue shaka-project#123", "Closes shaka-project#420")
3. Sign the Google CLA if you haven't (https://cla.developers.google.com)

You may delete this comment from the PR description.
-->