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/OneOperation.java

@@ -0,0 +1,10 @@
+package sh.okx.rankup.requirements.operation;
+
+import sh.okx.rankup.requirements.ReducerOperation;
+
+public class OneOperation extends ReducerOperation {
+  @Override
+  public boolean check(boolean a, boolean b) {
+    return (a && !b) || (b && !a);
+  }
+}