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/requirement/XpLevelRequirement.java

@@ -0,0 +1,36 @@
+package sh.okx.rankup.requirements.requirement;
+
+import org.bukkit.entity.Player;
+import sh.okx.rankup.Rankup;
+import sh.okx.rankup.requirements.DeductibleRequirement;
+import sh.okx.rankup.requirements.Requirement;
+
+public class XpLevelRequirement extends DeductibleRequirement {
+  public XpLevelRequirement(Rankup plugin, String name) {
+    super(plugin, name);
+  }
+
+  protected XpLevelRequirement(Requirement clone) {
+    super(clone);
+  }
+
+  @Override
+  public boolean check(Player player) {
+    return player.getLevel() >= getValueInt();
+  }
+
+  @Override
+  public void apply(Player player) {
+    player.setLevel(player.getLevel() - getValueInt());
+  }
+
+  @Override
+  public double getRemaining(Player player) {
+    return Math.max(0, getValueInt() - player.getLevel());
+  }
+
+  @Override
+  public Requirement clone() {
+    return new XpLevelRequirement(this);
+  }
+}