Allow delete on user-defined value types

[chriseth]

We do not allow any operator on user defined value types, but delete is special: It is implicitly used when we remove an array element and variables of user-defined value type have a default value - so

delete x; // x is of type Type

is essentially equivalent to

Type y;
x = y;

which means we already implicitly use it and it would be confusing to allow re-defining delete to something else.