Remove option to use default plugin from DatabaseUserExtension

Always specifying explicitly or otherwise intentionally calling with `null` for `plugin` and accepting the consequences is better.

Rationale: the Firebird setup on GitHub Actions has Legacy_UserManager as the default, and this results in breaking tests because Jaybird will by default only try Srp256 and Srp to connect.

Author: mrotteveel

src/test/org/firebirdsql/common/extension/DatabaseUserExtension.java

@@ -52,21 +52,6 @@ public static DatabaseUserExtension withDatabaseUser() {
         return new DatabaseUserExtension();
     }
 
-    /**
-     * Create a database user.
-     * <p>
-     * On Firebird 3.0 or higher this uses the default user manager plugin.
-     * </p>
-     *
-     * @param username
-     *         username
-     * @throws SQLException
-     *         For errors creating the user.
-     */
-    public void createUser(String username, String password) throws SQLException {
-        createUser(username, password, null);
-    }
-
     /**
      * Create a database user with the specified username and plugin.
      * <p>