Remove new platform members from OSPlatform

[terrajobst]

We decided in #40111 (comment) to not use RuntimeInformation and OSPlatform moving forward. Thus, we should remove these members that we added in the recent previews:

namespace System.Runtime.InteropServices
{
    public readonly partial struct OSPlatform
    {
-       public static System.Runtime.InteropServices.OSPlatform Android { get; }
-       public static System.Runtime.InteropServices.OSPlatform Browser { get; }
        public static System.Runtime.InteropServices.OSPlatform FreeBSD { get; }
-       public static System.Runtime.InteropServices.OSPlatform iOS { get; }
        public static System.Runtime.InteropServices.OSPlatform Linux { get; }
-       public static System.Runtime.InteropServices.OSPlatform macOS { get; }
-       [System.ComponentModel.EditorBrowsable(System.ComponentModel.EditorBrowsableState.Never)]
        public static System.Runtime.InteropServices.OSPlatform OSX { get; }
-       public static System.Runtime.InteropServices.OSPlatform tvOS { get; }
-       public static System.Runtime.InteropServices.OSPlatform watchOS { get; }
        public static System.Runtime.InteropServices.OSPlatform Windows { get; }
    }
}

/cc @jeffhandley @buyaa-n

[adamsitnik]

Fixed by #40373