afs: Remove dummy_PSetAcl()

The function dummy_PSetAcl() is not referenced anywhere within the
source tree.

Remove the function dummy_PSetAcl().

The commit: 'doxygen-comments-20081010' (4b72f8765c)
introduced the function, but dummy_PSetAcl() has never been referenced
or defined within a header file.

This function is flagged due to a missing prototype when building
against a Linux 6.8 kernel (which sets the -Wmissing-declarations and
-Wmissing-prototypes compiler flags as default). Linux 6.8 commit:
 'Makefile.extrawarn: turn on missing-prototypes globally' (0fcb70851f).

When building against a kernel with CONFIG_WERROR=y, the build fails.

Reviewed-on: https://gerrit.openafs.org/15618
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 05ca98687d882c72915e91430e29a4f2a36c502c)

Change-Id: Ie4420a365e07d2d8564530e004df75330e86bea5
Reviewed-on: https://gerrit.openafs.org/15687
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>

src/afs/afs_pioctl.c

@@ -1426,12 +1426,6 @@ DECL_PIOCTL(PGetFID)
  *
  * \post Changed ACL, via direct writing to the wire
  */
-int
-dummy_PSetAcl(char *ain, char *aout)
-{
-    return 0;
-}
-
 DECL_PIOCTL(PSetAcl)
 {
     afs_int32 code;