remove the empty Boolean.swift file from the stdlib, NFC.

Author: lattner

stdlib/public/core/Boolean.swift

@@ -28,7 +28,6 @@ set(SWIFTLIB_ESSENTIAL
   AssertCommon.swift
   BidirectionalCollection.swift
   Bool.swift
-  Boolean.swift
   BridgeObjectiveC.swift
   BridgeStorage.swift
   Builtin.swift