Get rid of empty IrBase class.

It isn't useful in any way now that the GC is out of the
picture, and the existance of a "dummy" toChars() method is
annoying when debugging.

ir/irstruct.h

@@ -26,7 +26,7 @@ struct StructInitializer;
 
 // represents a struct or class
 // it is used during codegen to hold all the vital info we need
-struct IrStruct : IrBase
+struct IrStruct
 {
     /// Constructor.
     IrStruct(AggregateDeclaration* agg);