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/requirements/Operation.java

@@ -0,0 +1,7 @@
+package sh.okx.rankup.requirements;
+
+import java.util.List;
+
+public abstract class Operation {
+  public abstract boolean check(List<Boolean> booleans);
+}