Remove Xunit.PlatformID enum

[justinvp]

Now that dotnet/corefx#12284 has been merged, we can delete PlatformID (replaced by TestPlatforms).

I did a search on GitHub and while there are some repos besides corefx using Microsoft.xunit.netcore.extensions (e.g. wcf, corefxlab), none that I could find are using PlatformID. (There may be private repos that need to be updated at some point that I can't see).

The following open corefx PRs are still using PlatformID in some way. I've left comments in each about the rename:

• Adding TLS Alerts corefx#11489
• Add tests for System.ComponentModel corefx#12238
• Port missing X509Certificate members corefx#12240
• Move System.Net.Sockets to netstandard 1.7 corefx#12249

Fixes #999

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[justinvp]

cc: @weshaggard

[weshaggard]

@justinvp there are other repo's that are using this right now as well. At least the WCF repo is so I would prefer we give this remove a little more time.

[weshaggard]

Sorry I didn't read you full description. Thanks for looking into other usages and if WCF isn't using this then I'm good with going ahead and removing it.