Remove old Windows 7 support code

[kennykerr]

As a reminder, whatever Windows 7 compatibility was previously provided is still available via older versions of the C++/WinRT nuget package.

Fixes: #1344

[kennykerr]

I've also removed tests that rely on GNU to build since their import libs don't include some Windows 8 exports. We can decide what to ultimately do about that here: #1341

The cppwinrt compiler continues to be cross compiled and both MSVC and Clang are tested on Windows.

[alvinhochun]

I believe the mingw-w64 link errors can be fixed by adding combase to the link libraries.

Edit: It probably should be runtimeobject instead... but will have to test it to be sure.
Yeah probably, I did add that for test and old_tests.