No need to suppress finalize for sealed classes

Since these classes are sealed and do not have finalizers, they will
never be put in the finalizer queue so there's no need for this call.

Author: haacked

src/GitHub.App/Caches/ImageCache.cs

@@ -239,7 +239,6 @@ void Dispose(bool disposing)
         public void Dispose()
         {
             Dispose(true);
-            GC.SuppressFinalize(this);
         }
 
         class UriComparer : IEqualityComparer<Uri>

src/GitHub.App/Caches/LoginCache.cs

@@ -78,7 +78,6 @@ void Dispose(bool disposing)
         public void Dispose()
         {
             Dispose(true);
-            GC.SuppressFinalize(this);
         }
     }
 }

src/GitHub.App/SampleData/SampleViewModels.cs

@@ -300,7 +300,6 @@ public string Title
 
         public void Dispose()
         {
-            GC.SuppressFinalize(this);
         }
 
         public IObservable<AuthenticationResult> LogIn(string usernameOrEmail, string password)