patch 8.2.4241: some type casts are redundant

Problem:    Some type casts are redundant.
Solution:   Remove the type casts. (closes #9643)
diff --git a/src/optionstr.c b/src/optionstr.c
index 4aa7cef..8d74ba0 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -181,7 +181,7 @@
 	    set_vim_var_string(VV_OPTION_OLDLOCAL, oldval, -1);
 	}
 	apply_autocmds(EVENT_OPTIONSET,
-		       (char_u *)get_option_fullname(opt_idx), NULL, FALSE,
+		       get_option_fullname(opt_idx), NULL, FALSE,
 		       NULL);
 	reset_v_option_vars();
     }