patch 8.2.4407: Vim9: some code not covered by tests

Problem:    Vim9: some code not covered by tests.
Solution:   Add more tests.  Avoid giving two errors.  Remove dead code.

src/testdir/test_vim9_cmd.vim

@@ -1528,6 +1528,17 @@ def Test_lockvar()
   assert_equal({a: 7, b: 5}, d)
 
   var lines =<< trim END
+      vim9script
+      g:bl = 0z1122
+      lockvar g:bl
+      def Tryit()
+        g:bl[1] = 99
+      enddef
+      Tryit()
+  END
+  v9.CheckScriptFailure(lines, 'E741:', 1)
+
+  lines =<< trim END
       vim9script
       var theList = [1, 2, 3]
       def SetList()