remove vestigial reference to 'sia'

remove vestigial reference to 'sia'

Commit 21006bb844a3 oops-sias-dead-20080906 removed references to 'sia'
from the tree, but overlooked the reference in the clean2 rule.  This
results in a harmless error from 'make clean', or any target with
'clean' as a dependency:

  /bin/sh: line 0: cd: sia: No such file or directory
  make[1]: [clean2] Error 1 (ignored)

Remove this last reference to eliminate the error.

Change-Id: Ic2827998e3b272acc7714238d0755c5ec2ad2b95
Reviewed-on: https://gerrit.openafs.org/14890
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>