Remove contrib/start-scripts/osx/.
Since those scripts haven't worked at all in macOS releases of 2014
and later, and aren't the recommended way to do it on any release
since 2005, there seems little point carrying them into the future.
It's very unlikely that anyone would be installing PG >= 11 on a
macOS release where they couldn't use contrib/start-scripts/macos/.
Discussion: https://postgr.es/m/31338.
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