CLDR-13285: Simplify plural logic for is, ceb, fil, tl, sl, kw

[roozbehp]

The new rules are mathematically identical, but are simpler and may be faster to calculate.

Checklist

• [x] Issue filed: https://unicode-org.atlassian.net/browse/CLDR-13285
• [x] Updated PR title and link in previous line to include Issue number

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[macchiati]

We should to have some tests to verify that the desired values are the same (for reasonable range of numbers).

Otherwise it is hard to make sure that results are exactly the same.