remove stats system

improve operations and make them extendable more easily and let other plugins add operations, like requirements 
move requirements/operations out of ranks class as they will be used in prestiges

src/main/java/sh/okx/rankup/prestige/Prestige.java

@@ -0,0 +1,4 @@
+package sh.okx.rankup.prestige;
+
+public class Prestige {
+}