Remove Interface is implemented by 1+ Objects validation

Author: mjmahone

spec/Section 3 -- Type System.md

@@ -1013,7 +1013,6 @@ Interface types have the potential to be invalid if incorrectly defined.
    type; no two fields may share the same name.
 3. Each field of an Interface type must not have a name which begins with the
    characters {"__"} (two underscores).
-4. An Interface type must be implemented by at least one Object type.
 
 
 ### Interface Extensions