Remove unnecessary instantiation in root

Fixes: gh-7635

Author: lee-ext

core/src/main/java/org/springframework/security/access/expression/SecurityExpressionRoot.java

@@ -17,7 +17,6 @@
 
 import java.io.Serializable;
 import java.util.Collection;
-import java.util.HashSet;
 import java.util.Set;
 
 import org.springframework.security.access.PermissionEvaluator;
@@ -158,7 +157,6 @@ public void setDefaultRolePrefix(String defaultRolePrefix) {
 
 	private Set<String> getAuthoritySet() {
 		if (roles == null) {
-			roles = new HashSet<>();
 			Collection<? extends GrantedAuthority> userAuthorities = authentication
 					.getAuthorities();