Revert "FEATURE: Allow theme tests to be run in production (#12815)" (PR #12840)

This reverts commit 7217dcb67a59f9c47d655455b60b4e1d8dc30f28.

Precompiling test_helper.js is so expensive that it can make bootstrap fail on servers with limited resources (2GB RAM). We will find another way that doesn’t require much resources.

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