Remove GlobalSuppressions classes

The `GlobalSuppressions` classes were introduced in #de969e8 to suppress `CA1062` false positives, since the *Code Analysis* extension doesn't recognize `[NotNull]` on ancillary/remote assemblies. That issue has been resolved by introducing the `[ValidatedNotNull]` attribute as well, as per #b9e0151. Given that, the `GlobalSuppressions` classes can be removed.

Author: JeremyCaney