Implement delete function in Subscriber repository.

src/repository/subscriber.rs

@@ -29,4 +29,16 @@ impl SubscriberRepository {
         return SUBSCRIBERS.get(product_type).unwrap().iter()
             .map(|f| f.value().clone()).collect();
     }
+
+    pub fn delete(product_type: &str, url: &str) -> Option<Subscriber> {
+        if SUBSCRIBERS.get(product_type).is_none() {
+            SUBSCRIBERS.insert(String::from(product_type), DashMap::new());
+        }
+        let result = SUBSCRIBERS.get(product_type).unwrap()
+            .remove(url);
+        if !result.is_none() {
+            return Some(result.unwrap().1);
+        }
+        return None;
+    }
 }
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