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.
diff --git a/src/main/java/com/gitblit/models/UserModel.java b/src/main/java/com/gitblit/models/UserModel.java
index f8f7ed6..1d9e413 100644
--- a/src/main/java/com/gitblit/models/UserModel.java
+++ b/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;