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/viminfo.c b/src/viminfo.c
index 540422c..11a2946 100644
--- a/src/viminfo.c
+++ b/src/viminfo.c
@@ -251,7 +251,7 @@
 viminfo_readstring(
     vir_T	*virp,
     int		off,		    // offset for virp->vir_line
-    int		convert UNUSED)	    // convert the string
+    int		convert)	    // convert the string
 {
     char_u	*retval = NULL;
     char_u	*s, *d;