Fix invalid Unix locale to Flutter locale (BCP-47) mapping

[robert-ancell]

Flutter locales use the BCP-47 format (https://www.rfc-editor.org/rfc/bcp/bcp47.txt), and Unix locales use a POSIX format. There is not a perfect mapping between them, see https://wiki.openoffice.org/wiki/LocaleMapping

The current implementation is not correctly setting the appropriate optional fields, so remove that.

There are likely some Unix locales that should set these fields, but these are best fixed by adding special cases for them as they are discovered.

Fixes: https://github.com/flutter/flutter/issues/111341

Pre-launch Checklist

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• [x] I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• [x] I listed at least one issue that this PR fixes in the description above.
• [ ] I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I signed the [CLA].
• [x] All existing and new tests are passing.

[flutter-dashboard[bot]]

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[Hixie]

@robert-ancell Is this still on your radar?

[robert-ancell]

@robert-ancell Is this still on your radar?

Was forgotten about, but I've added a test now.