Drop a CS0618 pragma that had nothing left to suppress (#845)

The region was labelled "Obsolete settings" and wrapped in a disable of
CS0618, but SettingClass.As(T, Action) is a live, documented member and is
not marked obsolete. There are now zero [Obsolete] members anywhere in
Sources, so the pragma suppressed nothing while still being able to hide the
next member that becomes obsolete inside those forty lines.

TreatWarningsAsErrors is on, so with the pragma removed a genuinely obsolete
call in that region would fail the build. It compiles clean.

The two regions do not divide into obsolete and current -- they cover the two
ways a setting can be scoped, the callback form and the disposable form, both
supported. They now say so.

Author: Rafael-SOWNet

Sources/Tests/UnitTests/Core/Multithreading/SettingsAndThreads.cs

@@ -16,8 +16,10 @@ namespace AngouriMath.Tests.Core.Multithreading
     [Trait("Area", "Core")]
     public sealed class SettingsAndThreads
     {
-        #region Obsolete settings
-#pragma warning disable CS0618 // The obsolete method should still work!
+        // A setting can be scoped two ways: As(value, action) runs the action under it, and
+        // Set(value) returns a disposable that restores it. Both are supported; each region
+        // covers one of them.
+        #region Callback form -- As(value, action)
         [Fact]
         public void OSettingValue1()
         {
@@ -86,10 +88,9 @@ public void OWithExceptionInside()
                 }
             );
         }
-#pragma warning restore CS0618 // The obsolote method should still work!
         #endregion
 
-        #region Normal settings
+        #region Disposable form -- Set(value)
         [Fact]
         public void SettingValue1()
         {