Code cleanup and formatting for System.Memory files. (#17451)

* Fix IDE0034 C# expression can be simplified (use of default)

* Remove unnecessary using statements

* IDE0012 C# Name can be simplified. String -> string

* IDE0041  C# Null check can be simplified

* Fix code formatting (limited to Span/System.Memory files).

* Address PR feedback - IDE0012 Name can be simplified (String -> string)

* Add back the necessary using statements and fix typo

* Fix comment typo and add necessary using statements

2 files changed, 3 insertions, 3 deletions

diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/ConcurrentExclusiveSchedulerPair.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/ConcurrentExclusiveSchedulerPair.cs
index 469535c0bb..146e477e2c 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/ConcurrentExclusiveSchedulerPair.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/ConcurrentExclusiveSchedulerPair.cs
@@ -295,7 +295,7 @@ namespace System.Threading.Tasks
                     try
                     {
                         processingTask = new Task(thisPair => ((ConcurrentExclusiveSchedulerPair)thisPair).ProcessExclusiveTasks(), this,
-                            default(CancellationToken), GetCreationOptionsForTask(fairly));
+                            default, GetCreationOptionsForTask(fairly));
                         processingTask.Start(m_underlyingTaskScheduler);
                         // When we call Start, if the underlying scheduler throws in QueueTask, TPL will fault the task and rethrow
                         // the exception.  To deal with that, we need a reference to the task object, so that we can observe its exception.
@@ -322,7 +322,7 @@ namespace System.Threading.Tasks
                             try
                             {
                                 processingTask = new Task(thisPair => ((ConcurrentExclusiveSchedulerPair)thisPair).ProcessConcurrentTasks(), this,
-                                    default(CancellationToken), GetCreationOptionsForTask(fairly));
+                                    default, GetCreationOptionsForTask(fairly));
                                 processingTask.Start(m_underlyingTaskScheduler); // See above logic for why we use new + Start rather than StartNew
                             }
                             catch
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskCompletionSource.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskCompletionSource.cs
index 992e9db767..11ba26d89d 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskCompletionSource.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskCompletionSource.cs
@@ -315,7 +315,7 @@ namespace System.Threading.Tasks
         /// <exception cref="T:System.ObjectDisposedException">The <see cref="Task"/> was disposed.</exception>
         public bool TrySetCanceled()
         {
-            return TrySetCanceled(default(CancellationToken));
+            return TrySetCanceled(default);
         }
 
         // Enables a token to be stored into the canceled task