[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.

(cherry picked from commit 2759a71)

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