Use void.class to determine how the event handler type is defined

The dummy class USE_PARAMETER was necessary when the IEvent interface
still existed, as void.class could not be used as a Class<? extends
IEvent>. As no explicit reference to USE_PARAMETER should be present
anywhere, a proper deprecation would've made little sense.

core/src/main/java/dev/kske/eventbus/core/Event.java

@@ -30,13 +30,5 @@ public @interface Event {
 	 * @return the event type accepted by the handler
 	 * @since 1.0.0
 	 */
-	Class<?> value() default USE_PARAMETER.class;
-
-	/**
-	 * Signifies that the event type the handler listens to is determined by the type of its only
-	 * parameter.
-	 *
-	 * @since 0.0.3
-	 */
-	static final class USE_PARAMETER {}
+	Class<?> value() default void.class;
 }

core/src/main/java/dev/kske/eventbus/core/handler/ReflectiveEventHandler.java

@@ -3,7 +3,6 @@ package dev.kske.eventbus.core.handler;
 import java.lang.reflect.*;
 
 import dev.kske.eventbus.core.*;
-import dev.kske.eventbus.core.Event.USE_PARAMETER;
 
 /**
  * An event handler wrapping a method annotated with {@link Event} and executing it using
@@ -37,7 +36,7 @@ public final class ReflectiveEventHandler implements EventHandler {
 		boolean defPolymorphism, int defPriority) throws EventBusException {
 		this.listener	= listener;
 		this.method		= method;
-		useParameter	= annotation.value() == USE_PARAMETER.class;
+		useParameter	= annotation.value() == void.class;
 
 		// Check handler signature
 		if (method.getParameterCount() == 0 && useParameter)