Temporary remove OSX.1012.Amd64.Open and OSX.1012.Amd64 Helix queues

[echesakov]

Currently, the build agents in Azure DevOps run on OSX 10.13. This causes the following failures when a test job submitted to OSX 10.12 queue.

dlopen failed to open the libcoreclr.so with error dlopen(/Users/dotnet-bot/dotnetbuild/work/955a3b93-78a6-41fb-8696-8e73e9be05b2/Payload/libcoreclr.dylib, 6): Symbol not found: _syslog$DARWIN_EXTSN
  Referenced from: /Users/dotnet-bot/dotnetbuild/work/955a3b93-78a6-41fb-8696-8e73e9be05b2/Payload/libcoreclr.dylib (which was built for Mac OS X 10.13)
  Expected in: /usr/lib/libSystem.B.dylib
 in /Users/dotnet-bot/dotnetbuild/work/955a3b93-78a6-41fb-8696-8e73e9be05b2/Payload/libcoreclr.dylib

This PR temporary removes OSX.1012.Amd64.Open and OSX.1012.Amd64 from Helix target queues.

Related issue: https://github.com/dotnet/core-eng/issues/4856

[danmoseley]

@dotnet-bot test this please (fixed tests)

[MattGal]

LGTM, I'd also point out that we have a reasonable number of machines in OSX.1014.Amd64 now though, so you could consider at least having these on the Internal list if not both (assuming builds don't happen on OSX.1014.Amd64.Open, the 6 or so machines we have should do the trick.)

[am11]

Would it fix the error if add_compile_options(-mmacosx-version-min=10.12) is added after this line?

[echesakov]

Would it fix the error if add_compile_options(-mmacosx-version-min=10.12) is added after this line?

@am11 Thanks for pointing this out - I can try this option.

[echesakov]

@MattGal I'll test and add OSX.1014.Amd64.* queues in another PR. Thanks