Remove [Signal] attribute from events

- Remove event as a valid target of `SignalAttribute`
- Stop adding the `[Signal]` attribute to events in bindings_generator
- Make bindings_generator use the `EventHandler` suffix to be consistent with the C# source generator
- Remove obsolete comment about the signal's delegate name

modules/mono/glue/GodotSharp/GodotSharp/Core/Attributes/SignalAttribute.cs

@@ -2,6 +2,6 @@ using System;
 
 namespace Godot
 {
-    [AttributeUsage(AttributeTargets.Delegate | AttributeTargets.Event)]
+    [AttributeUsage(AttributeTargets.Delegate)]
     public class SignalAttribute : Attribute { }
 }