refactor: Get rid of unnecessary type cast

lib/encryption/encryption.dart

@@ -399,7 +399,7 @@ class Encryption {
     return encryptedPayload;
   }
 
-  Future<Map<String, dynamic>> encryptToDeviceMessage(
+  Future<Map<String, Map<String, Map<String, dynamic>>>> encryptToDeviceMessage(
       List<DeviceKeys> deviceKeys,
       String type,
       Map<String, dynamic> payload) async {

lib/encryption/olm_manager.dart

@@ -631,7 +631,7 @@ class OlmManager {
     return encryptedBody;
   }
 
-  Future<Map<String, dynamic>> encryptToDeviceMessage(
+  Future<Map<String, Map<String, Map<String, dynamic>>>> encryptToDeviceMessage(
       List<DeviceKeys> deviceKeys,
       String type,
       Map<String, dynamic> payload) async {

lib/src/client.dart

@@ -2617,8 +2617,10 @@ class Client extends MatrixApi {
 
       // Send with send-to-device messaging
       final data = await encryption.encryptToDeviceMessage(
-              deviceKeys, eventType, message)
-          as Map<String, Map<String, Map<String, dynamic>>>;
+        deviceKeys,
+        eventType,
+        message,
+      );
       eventType = EventTypes.Encrypted;
       await sendToDevice(
           eventType, messageId ?? generateUniqueTransactionId(), data);