[stdlib] OptionSet: Remove the primary associated type declaration

`Element` is expected to always be `Self`. `RawValue` would be a better choice for the primary type ("option set of UInt32"), but to avoid confusion, it seems better to just omit the declaration, at least for now.

Author: lorentey

stdlib/public/core/OptionSet.swift

@@ -83,7 +83,7 @@
 ///         print("Add more to your cart for free priority shipping!")
 ///     }
 ///     // Prints "You've earned free priority shipping!"
-public protocol OptionSet<Element>: SetAlgebra, RawRepresentable {
+public protocol OptionSet: SetAlgebra, RawRepresentable {
   // We can't constrain the associated Element type to be the same as
   // Self, but we can do almost as well with a default and a
   // constrained extension