Remove the -enable-union-import staging flag.

Swift SVN r25809

include/swift/Basic/LangOptions.h

@@ -109,9 +109,6 @@ namespace swift {
     /// it gets disabled when compiling the Swift core stdlib.
     bool EnableObjCAttrRequiresFoundation = true;
 
-    /// Enables importing of unions and other partially-importable types.
-    bool ImportUnions = true;
-    
     /// Should access control be respected?
     bool EnableAccessControl = true;