remove an unused import

samples/boot/oauth2authorizationserver/src/main/java/sample/JwkSetConfiguration.java

@@ -18,7 +18,6 @@ package sample;
 import java.math.BigInteger;
 import java.security.KeyFactory;
 import java.security.KeyPair;
-import java.security.Principal;
 import java.security.interfaces.RSAPublicKey;
 import java.security.spec.RSAPrivateKeySpec;
 import java.security.spec.RSAPublicKeySpec;