Delete several unused memory management constants

Change 412854593cf368006c18e6c0dc607a9ecd76a0e0, removed from
the code base the last usage of:
AFS_SALLOC_LOW_WATER (defined in afs/afs.h)
AFS_MALLOC_LOW_WATER (defined in config/afs_args.h)
AFS_MDALLOCSIZ (defined in config/afs_args.h)
This patch deletes these constants.

Change-Id: I1333aed508875e9b13dc3f36f3ff0d5eadfb2cfd
Reviewed-on: http://gerrit.openafs.org/11173
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>