Enable System.IO.Pipes test disabled against Issue#1840

System.IO.Pipes dependended on some new APISets introduced in WIndows10.
Since the OS was still under development, it was not available on all the images, making the tests flaky.
Now that the things are stable, enabling the tests again.

Author: Pallavi Taneja

src/System.IO.Pipes/tests/AnonymousPipesSimpleTest.cs

@@ -199,7 +199,6 @@ public static async Task ServerPInvokeChecks()
     }
 
     [Fact]
-    [ActiveIssue(1840, PlatformID.Windows)]
     public static async Task ClientPInvokeChecks()
     {
         using (AnonymousPipeServerStream server = new AnonymousPipeServerStream(PipeDirection.In, System.IO.HandleInheritability.None, 4096))

src/System.IO.Pipes/tests/NamedPipesSimpleTest.cs

@@ -355,7 +355,6 @@ public static async Task ClientPInvokeChecks()
     }
 
     [Fact]
-    [ActiveIssue(1840)]
     [PlatformSpecific(PlatformID.Windows)]
     public static void ClientServerMessages()
     {
@@ -407,7 +406,6 @@ public static void ClientServerMessages()
     }
 
     [Fact]
-    [ActiveIssue(1840, PlatformID.Windows)]
     public static async Task ServerCloneTests()
     {
         const string pipeName = "fooclone";
@@ -439,7 +437,6 @@ public static async Task ServerCloneTests()
     }
 
     [Fact]
-    [ActiveIssue(1840, PlatformID.Windows)]
     public static async Task ClientCloneTests()
     {
         const string pipeName = "fooClientclone";