RFR: 8290300: Use standard String-joining tools where applicable [v4]

[Naoto Sato]

On Mon, 18 Jul 2022 11:08:24 GMT, Сергей Цыпанов <duke at openjdk.org> wrote:

> For `Locale` all the call sites of `formatList()` never pass null, so the code in `if` block is never executed. I think we can delete this unused parts of the code in `Locale`,

Are you sure about this? `pattern` is derived from `LocaleResources.getLocaleName()` which could return `null`.

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PR: https://git.openjdk.org/jdk/pull/9513