[stdlib] LazySequenceProtocol: Take out duplicate where clause

(cherry picked from commit 655f085)

Author: lorentey

stdlib/public/core/LazySequence.swift

@@ -128,8 +128,7 @@
 /// and discards the resulting array. Instead, use `reduce` for summing 
 /// operations, or `forEach` or a `for`-`in` loop for operations with side 
 /// effects.
-public protocol LazySequenceProtocol<Elements>: Sequence
-where Elements.Element == Element {
+public protocol LazySequenceProtocol<Elements>: Sequence {
   /// A `Sequence` that can contain the same elements as this one,
   /// possibly with a simpler type.
   ///