afs_server: afs_SetServerPrefs() can never be called with null

afs_server: afs_SetServerPrefs() can never be called with null

The one and only call site never calls afs_SetServerPrefs() with a
null pointer, and some but not all of the paths through the #ifdefs
assume this.  Remove code that checks for this; it confuses both
humans and the static analyzer.

Change-Id: Ie3f7319cebde9ec3078af2defcebb80692aa95f2
Reviewed-on: http://gerrit.openafs.org/7912
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>