patch 9.1.0633: Compilation warnings with `-Wunused-parameter`

Problem:  Compilation warnings with `-Wunused-parameter`
Solution: Add the `UNUSED` macro where needed, and remove some
          superfluous ones (Dominique Pellé)

Change fixes these kind of warnings when building without the channel
feature:

```
eval.c:6122:15: warning: unused parameter ‘tv’ [-Wunused-parameter]
     typval_T *tv,
               ^
eval.c:6123:14: warning: unused parameter ‘tofree’ [-Wunused-parameter]
     char_u **tofree,
              ^
eval.c:6124:13: warning: unused parameter ‘numbuf’ [-Wunused-parameter]
     char_u *numbuf,
             ^
eval.c:6125:10: warning: unused parameter ‘composite_val’ [-Wunused-parameter]
     int  composite_val)
```

closes: #15378

Signed-off-by: Dominique Pellé <dominique.pelle@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/textprop.c b/src/textprop.c
index 83c42a3..fe0c8d2 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -2060,7 +2060,7 @@
  * prop_type_list([{bufnr}])
  */
     void
-f_prop_type_list(typval_T *argvars, typval_T *rettv UNUSED)
+f_prop_type_list(typval_T *argvars, typval_T *rettv)
 {
     buf_T *buf = NULL;