remove unused code.

src/DotNetCore.CAP/CAP.Builder.cs

@@ -34,17 +34,6 @@ namespace DotNetCore.CAP
             return this;
         }
 
-        /// <summary>
-        /// Adds a singleton service of the type specified in serviceType with an implementation
-        /// </summary>
-        private CapBuilder AddSingleton<TService, TImplementation>()
-            where TService : class
-            where TImplementation : class, TService
-        {
-            Services.AddSingleton<TService, TImplementation>();
-            return this;
-        }
-
         /// <summary>
         /// Add an <see cref="ICapPublisher"/>.
         /// </summary>