Fix unused import warning

Author: Phillip Webb

spring-core/src/main/java/org/springframework/core/ResolvableType.java

@@ -36,7 +36,6 @@
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ConcurrentReferenceHashMap;
-import org.springframework.util.ConcurrentReferenceHashMap.ReferenceType;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;