patch 8.2.1623: Vim9: using :call where it is not needed

Problem:    Vim9: using :call where it is not needed.
Solution:   Remove :call. (closes #6892)
diff --git a/src/testdir/test_maparg.vim b/src/testdir/test_maparg.vim
index 3b61ff6..b2ace6f 100644
--- a/src/testdir/test_maparg.vim
+++ b/src/testdir/test_maparg.vim
@@ -90,7 +90,7 @@
   let three: string = maparg('{', 'n', 0)
   assert_equal('w', three)
   let four: dict<any> = maparg('{', 'n', 0, 1)
-  call assert_equal(['{', 'w', 'n'], [four.lhs, four.rhs, four.mode])
+  assert_equal(['{', 'w', 'n'], [four.lhs, four.rhs, four.mode])
   nunmap {
 enddef