Delete Gpio code

[joperezr]

All of the System.Device.Gpio source code and samples have migrated to dotnet/iot repo. Removing all of the code from corefx lab and add a few .md files to point people to the new home.

[ahsonkhan]

Instead of adding new READMEs and keeping the folders around, can you instead add this to a heading to the root README after archived projects?

https://github.com/dotnet/corefxlab#archived-projects

[joperezr]

I think I'd still prefer to keep these .mds around, as I think it is more likely for people to find this and navigate to the new location than if we have them listed in the main README.md file in section of archived projects

[ahsonkhan]

It seems odd to have, an essentially empty directory, with the project format. But I don't feel strongly about it.

[joperezr]

I'll keep them for now, given that I do believe they will be useful, but if I get more feedback saying to remove them, or you feel strongly about removing them, then I'll go ahead and remove.

[ahsonkhan]

The projects need to be removed from the corefxlab.sln. That is what's causing CI to fail.

[ahsonkhan]

https://ci.dot.net/job/dotnet_corefxlab/job/master/job/osx10.12_debug_prtest/2914/consoleFull#173922874233fe3402-0c0f-45a7-a707-47164965bab5

16:09:21 Telemetry statement: https://aka.ms/dotnet-cli-telemetry
16:09:29 
/Users/dotnet-bot/j/workspace/dotnet_corefxlab/master/osx10.12_debug_prtest/dotnetcli/sdk/3.0.100-preview-009844/NuGet.targets(499,5): error : Could not find a part of the path '/tmp/NuGetScratch/6d9749c7-fd7a-4c7a-8a33-db9dc5d29f0f'. [/Users/dotnet-bot/j/workspace/dotnet_corefxlab/master/osx10.12_debug_prtest/src/System.Text.Http/System.Text.Http.csproj]
16:09:30 
Failed to restore packages.
16:09:30 

cc @MattGal, we are seeing similar, intermittent, package restore errors as before:
https://github.com/dotnet/core-eng/issues/4721

[MattGal]

@ahsonkhan the core-eng issue you linked here was about Windows machines, the logs in the OSX restore failure you've pasted match NuGet/Home#7341 but we still don't know that actual cause of that one. If you manage to identify any correlation between specific machines (file permissions / ownership might be at play here, from the Nuget thread) that would help a lot.