gh-137976: fix available_timezones to prevent reporting an invalid IANA zone name: localtime

[ntindle]

When comparing timezones on my Mac and GitHub Actions runners, I found that localtime was found in the timezone list for the runners, while not on my Mac. This is caused by the link (https://linuxman7.com/linux/man-pages/man5/localtime.5.html) that some systems make to the local timezone for easy lookup.

I've removed it from the list, similar to how posixrules is removed.

Note that Factory also exists but is in both lists so I didn't remove it.

• Issue: zoneinfo.available_timezones() sometimes include 'localtime' #137976

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