Remove duplicate import of class SecureRandom

Fixes the build that was broken by cherry-picking commit 2be2c2,
which resulted in an import collision on the `SecureRandom` class.

Author: fzs

src/main/java/com/gitblit/models/UserModel.java

@@ -17,7 +17,6 @@
 
 import java.io.Serializable;
 import java.security.Principal;
-import java.security.SecureRandom;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashSet;