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/window.c b/src/window.c
index db0bb1b..7ca29d4 100644
--- a/src/window.c
+++ b/src/window.c
@@ -1708,7 +1708,7 @@
     int
 make_windows(
     int		count,
-    int		vertical UNUSED)  // split windows vertically if TRUE
+    int		vertical)  // split windows vertically if TRUE
 {
     int		maxcount;
     int		todo;
@@ -3486,7 +3486,7 @@
     win_T *
 winframe_remove(
     win_T	*win,
-    int		*dirp UNUSED,	// set to 'v' or 'h' for direction if 'ea'
+    int		*dirp,		// set to 'v' or 'h' for direction if 'ea'
     tabpage_T	*tp,		// tab page "win" is in, NULL for current
     frame_T	**unflat_altfr)	// if not NULL, set to pointer of frame that got
 				// the space, and it is not flattened
@@ -4855,8 +4855,8 @@
  */
     static int
 leave_tabpage(
-    buf_T	*new_curbuf UNUSED,    // what is going to be the new curbuf,
-				       // NULL if unknown
+    buf_T	*new_curbuf,		// what is going to be the new curbuf,
+					// NULL if unknown
     int		trigger_leave_autocmds)
 {
     tabpage_T	*tp = curtab;
@@ -4908,7 +4908,7 @@
     static void
 enter_tabpage(
     tabpage_T	*tp,
-    buf_T	*old_curbuf UNUSED,
+    buf_T	*old_curbuf,
     int		trigger_enter_autocmds,
     int		trigger_leave_autocmds)
 {
@@ -7855,7 +7855,7 @@
 }
 
     int
-get_tab_number(tabpage_T *tp UNUSED)
+get_tab_number(tabpage_T *tp)
 {
     int		i = 1;
     tabpage_T	*t;