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/RequirementRegistry.java

@@ -0,0 +1,23 @@
+package sh.okx.rankup.requirements;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class RequirementRegistry {
+  private Set<Requirement> requirements = new HashSet<>();
+
+  public void addRequirement(Requirement requirement) {
+    requirements.add(requirement);
+  }
+
+  public Requirement newRequirement(String name, String value) {
+    for (Requirement requirement : requirements) {
+      if (requirement.getName().equalsIgnoreCase(name)) {
+        Requirement newRequirement = requirement.clone();
+        newRequirement.setValue(value);
+        return newRequirement;
+      }
+    }
+    return null;
+  }
+}