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/DeductibleRequirement.java

@@ -0,0 +1,24 @@
+package sh.okx.rankup.requirements;
+
+import org.bukkit.entity.Player;
+import sh.okx.rankup.Rankup;
+
+public abstract class DeductibleRequirement extends Requirement {
+  public DeductibleRequirement(Rankup plugin, String name) {
+    super(plugin, name);
+  }
+
+  protected DeductibleRequirement(Requirement clone) {
+    super(clone);
+  }
+
+  /**
+   * Apply the effect of this requirement to the player.
+   * For money, this could be taking money away from the player.
+   * You can assume that <code>Requirement#check(Player)</code> has been called,
+   * and has returned true immediately prior to this.
+   *
+   * @param player the player to take from
+   */
+  public abstract void apply(Player player);
+}