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.

Change-Id: I9583c8e5adc4106848a437d81306000490787ef3
Reviewed-on: https://gerrit.openafs.org/11938
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>