babel-preset-env: do not repeat, that targets.esmodules discards targets.browsers

https://babeljs.io/docs/en/babel-preset-env#targetsesmodules says:

> When specifying this option, the browsers field will be ignored. You can use this approach in combination with to conditionally serve smaller scripts to users (https://jakearchibald.com/2017/es-modules-in-browsers/#nomodule-for-backwards-compatibility).
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> Please note: when specifying the esmodules target, browsers targets will be ignored.

As this text says twice, that targets.browsers will be ignored, deduplicate it.