Do not set default AFS_SYSCALL

Do not set default AFS_SYSCALL

Currently, afs_args.h will define an AFS_SYSCALL value by default (31)
if the current platform does not define an AFS_SYSCALL value on its
own (via its param.h info).

This is dangerous, since if a platform does not define an AFS_SYSCALL,
or if it happens to not be defined for any reason, some code may try
to call syscall 31, which could be anything.

So get rid of this. If this breaks the build on any platform, then
that platform should define AFS_SYSCALL in its own platform-specific
header, or get rid of the problematic AFS_SYSCALL usage.

Reviewed-on: https://gerrit.openafs.org/11938
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 0a896b93c86e86f5b438880ef1634b4e39ee5779)

Change-Id: Ifcc17d5e26fde01302c43331ffdf005b6b9c3bd3
Reviewed-on: https://gerrit.openafs.org/13093
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>