Investigate trimming of static interface methods when the interface is not used

Currently linker will keep the interface if there's a method which implements one of the static interface methods on it and that implementation method is kept. This is unlike non-static interface methods where the same situation still allows the linker to remove the interface.

Sample:
```C#
void Main()
{
TestClass.Method (); // Use the implementation method only
}

[Kept] // ??? Why
interface IStaticIFace
{
[Kept] // ??? Why
static abstract Method();
}

[Kept] // This is correct
class TestClass : IStaticIFace
{
[Kept] // This is correct
public static abstract Method() {};
}
```