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/strings.c b/src/strings.c
index 6b2ff0a..20ca65d 100644
--- a/src/strings.c
+++ b/src/strings.c
@@ -1037,7 +1037,7 @@
  * Implementation of "byteidx()" and "byteidxcomp()" functions
  */
     static void
-byteidx_common(typval_T *argvars, typval_T *rettv, int comp UNUSED)
+byteidx_common(typval_T *argvars, typval_T *rettv, int comp)
 {
     rettv->vval.v_number = -1;