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/misc2.c b/src/misc2.c
index c07ed80..b5044fb 100644
--- a/src/misc2.c
+++ b/src/misc2.c
@@ -72,7 +72,7 @@
  * Get the screen position of character col with a coladd in the cursor line.
  */
     int
-getviscol2(colnr_T col, colnr_T coladd UNUSED)
+getviscol2(colnr_T col, colnr_T coladd)
 {
     colnr_T	x;
     pos_T	pos;