patch 8.2.4940: some code is never used

Problem:    Some code is never used.
Solution:   Remove dead code.  Add a few more test cases.
diff --git a/src/proto/vim9expr.pro b/src/proto/vim9expr.pro
index aa84509..233cfa2 100644
--- a/src/proto/vim9expr.pro
+++ b/src/proto/vim9expr.pro
@@ -2,7 +2,7 @@
 int generate_ppconst(cctx_T *cctx, ppconst_T *ppconst);
 void clear_ppconst(ppconst_T *ppconst);
 int compile_member(int is_slice, int *keeping_dict, cctx_T *cctx);
-int compile_load_scriptvar(cctx_T *cctx, char_u *name, char_u *start, char_u **end, int error);
+int compile_load_scriptvar(cctx_T *cctx, char_u *name, char_u *start, char_u **end);
 int compile_load(char_u **arg, char_u *end_arg, cctx_T *cctx, int is_expr, int error);
 char_u *to_name_end(char_u *arg, int use_namespace);
 char_u *to_name_const_end(char_u *arg);