Remove unused StorageBackend protocol

Sources/Cache/DiskStorage.swift

@@ -38,7 +38,7 @@ public enum DiskStorage {
     /// You can config a `DiskStorage.Backend` in its initializer by passing a `DiskStorage.Config` value.
     /// or modifying the `config` property after it being created. `DiskStorage` will use file's attributes to keep
     /// track of a file for its expiration or size limitation.
-    public class Backend<T: DataTransformable>: StorageBackend {
+    public class Backend<T: DataTransformable> {
         /// The config used for this disk storage.
         public var config: Config
 

Sources/Cache/MemoryStorage.swift

@@ -41,7 +41,7 @@ public enum MemoryStorage {
     /// date. When retrieved, if the target item is already expired, it will be recognized as it does not
     /// exist in the storage. The `MemoryStorage` also contains a scheduled self clean task, to evict expired
     /// items from memory.
-    public class Backend<T: CacheCostCalculable>: StorageBackend {
+    public class Backend<T: CacheCostCalculable> {
         let storage = NSCache<NSString, StorageObject<T>>()
         var keys = Set<String>()
 

Sources/Cache/Storage.swift

@@ -73,19 +73,6 @@ public enum StorageExpiration {
     }
 }
 
-protocol StorageBackend {
-    associatedtype ValueType
-    associatedtype KeyType
-    func store(
-        value: ValueType,
-        forKey key: KeyType,
-        expiration: StorageExpiration?) throws
-    func value(forKey key: KeyType) throws -> ValueType?
-    func remove(forKey key: String) throws
-    func removeAll() throws
-    func isCached(forKey key: String) -> Bool
-}
-
 /// Represents types which cost in memory can be calculated.
 public protocol CacheCostCalculable {
     var cacheCost: Int { get }