layers: Remove unused unistd.h includes

[pinotree]

The sources seem to build fine even without them (at least on Linux), so drop these OS-specific includes.

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Author pinotree not on autobuild list. Waiting for curator authorization before starting CI build.

[CLAassistant]

All committers have signed the CLA.

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Author pinotree not on autobuild list. Waiting for curator authorization before starting CI build.

[pinotree]

I'm curious how you noticed this

Because of my GNU/Hurd porting (#8177): rather than adding more ifdefs, I decided to drop the ones that were not needed (at least as it seemed).

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[spencer-lunarg]

CI Vulkan-ValidationLayers build # 16879 failed.

The error logs are showing strange github/internet issue

ssh: Could not resolve hostname github.com: Temporary failure in name resolution

will wait a bit and kick it off again and get this in soon

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CI Vulkan-ValidationLayers build # 16882 running.

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CI Vulkan-ValidationLayers build # 16886 running.

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