Clean up AsyncHelpers.cs after #2371

- Add trailing newline (was missing in the squashed PR)
- Remove stale "Adapted from Rebus" header line — the Rebus-derived
  CustomSynchronizationContext was removed in #2371

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: alexeyzimarev

src/RestSharp/AsyncHelpers.cs

@@ -12,7 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 //
-// Adapted from Rebus
 
 namespace RestSharp;
 
@@ -30,4 +29,4 @@ static class AsyncHelpers {
     public static T RunSync<T>(Func<Task<T>> func) {
         return Task.Run(func).GetAwaiter().GetResult();
     }
-}
+}