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/vim9cmds.c b/src/vim9cmds.c
index 694bb33..c7fa60a 100644
--- a/src/vim9cmds.c
+++ b/src/vim9cmds.c
@@ -1639,7 +1639,7 @@
  * Compile "catch {expr}".
  */
     char_u *
-compile_catch(char_u *arg, cctx_T *cctx UNUSED)
+compile_catch(char_u *arg, cctx_T *cctx)
 {
     scope_T	*scope = cctx->ctx_scope;
     garray_T	*instr = &cctx->ctx_instr;
@@ -1923,7 +1923,7 @@
  * compile "throw {expr}"
  */
     char_u *
-compile_throw(char_u *arg, cctx_T *cctx UNUSED)
+compile_throw(char_u *arg, cctx_T *cctx)
 {
     char_u *p = skipwhite(arg);