Fix checkstyle violation

Author: snicoll

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationprocessor/metadata/Metadata.java

@@ -26,7 +26,6 @@
 import org.hamcrest.collection.IsMapContaining;
 
 import org.springframework.boot.configurationprocessor.metadata.ItemMetadata.ItemType;
-import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 
 /**