fix: create clients only if necessary

Author: imabhichow

src/main/java/software/amazon/encryption/s3/materials/KmsKeyring.java

@@ -14,7 +14,6 @@
 import software.amazon.awssdk.services.kms.model.EncryptResponse;
 import software.amazon.awssdk.services.kms.model.GenerateDataKeyRequest;
 import software.amazon.awssdk.services.kms.model.GenerateDataKeyResponse;
-import software.amazon.awssdk.services.s3.S3Client;
 import software.amazon.awssdk.services.s3.model.GetObjectRequest;
 import software.amazon.awssdk.services.s3.model.S3Request;
 import software.amazon.encryption.s3.S3EncryptionClient;