Properly unregister plugin channels when a plugin gets disabled.

By: Meaglin <meaglin.wasabi@gmail.com>

paper-api/src/main/java/org/bukkit/plugin/SimplePluginManager.java

@@ -416,6 +416,13 @@ public final class SimplePluginManager implements PluginManager {
             } catch (Throwable ex) {
                 server.getLogger().log(Level.SEVERE, "Error occurred (in the plugin loader) while unregistering events for " + plugin.getDescription().getFullName() + " (Is it up to date?): " + ex.getMessage(), ex);
             }
+            
+            try {
+                server.getMessenger().unregisterIncomingPluginChannel(plugin);
+                server.getMessenger().unregisterOutgoingPluginChannel(plugin);
+            } catch(Throwable ex) {
+                server.getLogger().log(Level.SEVERE, "Error occurred (in the plugin loader) while unregistering plugin channels for " + plugin.getDescription().getFullName() + " (Is it up to date?): " + ex.getMessage(), ex);
+            }
         }
     }