remove virtual from interface declarations (#1639)

It's not needed, and it's likely to lead to reader confusion.

Author: BillWagner

csharp/tutorials/mixins-with-interfaces/IBlinkingLight.cs

@@ -8,7 +8,7 @@ namespace mixins_with_interfaces
     // <SnippetBlinkingLight>
     public interface IBlinkingLight : ILight
     {
-        public virtual async Task Blink(int duration, int repeatCount)
+        public async Task Blink(int duration, int repeatCount)
         {
             Console.WriteLine("Using the default interface method for IBlinkingLight.Blink.");
             for (int count = 0; count < repeatCount; count++)

csharp/tutorials/mixins-with-interfaces/ILight.cs

@@ -22,7 +22,7 @@ public interface ILight
         void SwitchOn();
         void SwitchOff();
         bool IsOn();
-        public virtual PowerStatus Power() => PowerStatus.NoPower;
+        public PowerStatus Power() => PowerStatus.NoPower;
     }
     // </SnippetILightInterface>
 }

csharp/tutorials/mixins-with-interfaces/ITimerLight.cs

@@ -8,7 +8,7 @@ namespace mixins_with_interfaces
     // <SnippetTimerLightFinal>
     public interface ITimerLight : ILight
     {
-        public virtual async Task TurnOnFor(int duration)
+        public async Task TurnOnFor(int duration)
         {
             Console.WriteLine("Using the default interface method for the ITimerLight.TurnOnFor.");
             SwitchOn();

csharp/tutorials/mixins-with-interfaces/mixins-with-interfaces.csproj

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <TargetFramework>netcoreapp3.0</TargetFramework>
-    <RootNamespace>virtual_interface_methods</RootNamespace>
+    <RootNamespace>mixins_with_interfaces</RootNamespace>
   </PropertyGroup>
 
 </Project>