server: Fix new compiler / clippy warnings (#1377)

## Motivation

Fixes the CI failure in #1376.

Author: svix-jplatte

server/svix-server/src/core/cache/memory.rs

@@ -132,8 +132,6 @@ mod tests {
         }
     }
 
-    #[derive(Deserialize, Serialize, Debug, PartialEq)]
-    struct StringTestVal(String);
     string_kv_def!(StringTestKey);
     impl StringTestKey {
         fn new(id: String) -> StringTestKey {

server/svix-server/src/core/cache/redis.rs

@@ -67,7 +67,7 @@ impl CacheBehavior for RedisCache {
     async fn delete<T: CacheKey>(&self, key: &T) -> Result<()> {
         let mut pool = self.redis.get().await?;
 
-        pool.del(key.as_ref()).await?;
+        let _: () = pool.del(key.as_ref()).await?;
 
         Ok(())
     }
@@ -103,8 +103,6 @@ mod tests {
         }
     }
 
-    #[derive(Deserialize, Serialize, Debug, PartialEq)]
-    struct StringTestVal(String);
     string_kv_def!(StringTestKey);
     impl StringTestKey {
         fn new(id: String) -> StringTestKey {

server/svix-server/tests/it/redis_queue.rs

@@ -39,7 +39,7 @@ async fn test_many_queue_consumers_inner(prefix: &str, delay: Option<Duration>)
         let mut conn = pool.get().await.unwrap();
 
         let _: () = conn
-            .del(&format!("{prefix}{{queue}}_svix_v3_main"))
+            .del(format!("{prefix}{{queue}}_svix_v3_main"))
             .await
             .unwrap();
     }