Remove the system property check for uuidOverride (#459)

as it caused a server timeout...
https://pastebin.com/z5ktU8r3

worldedit-core/src/main/java/com/sk89q/worldedit/session/SessionManager.java

@@ -243,13 +243,8 @@ public class SessionManager {
      * @return the key object
      */
     protected UUID getKey(SessionKey key) {
-        String forcedKey = System.getProperty("worldedit.session.uuidOverride");
-        if (forcedKey != null) {
-            return UUID.fromString(forcedKey);
-        } else {
         return key.getUniqueId();
     }
-    }
 
     /**
      * Remove the session for the given owner if one exists.