(maint) Add rule to error on unnecessary usings

This is just good practice.

Author: gep13

src/.editorconfig

@@ -246,6 +246,9 @@ dotnet_diagnostic.SA1633.severity = none
 # IDE1006: Naming Styles
 dotnet_diagnostic.IDE1006.severity = none
 
+# IDE0005: Using directive is unnecessary.
+dotnet_diagnostic.IDE0005.severity = error
+
 [*.{cs,vb}]
 dotnet_style_operator_placement_when_wrapping = beginning_of_line
 tab_width = 4

src/Cake.AppVeyor.Tests/AppVeyorTests.cs

@@ -1,7 +1,6 @@
 using Xunit;
 using System;
 using System.Linq;
-using Cake.AppVeyor;
 using Cake.AppVeyor.Fakes;
 
 namespace Cake.AppVeyor.Tests

src/Cake.AppVeyor/AppVeyorClient.cs

@@ -1,9 +1,5 @@
 using System;
-using System.Collections.Generic;
 using System.Net.Http;
-using System.Net.Http.Headers;
-using System.Threading;
-using System.Threading.Tasks;
 using Refit;
 
 namespace Cake.AppVeyor

src/Cake.AppVeyor/AppVeyorSettings.cs

@@ -1,8 +1,3 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using Cake.Core.Annotations;
-
 namespace Cake.AppVeyor
 {
     /// <summary>