libuafs: Don't #define user

libuafs: Don't #define user

libuafs used to #define user to usr_user, so that any references to
'struct user' would become 'struct usr_user'. However, none of the
kernel code uses struct user, and this #define conflicts with the
definitions in sys/user.h on Linux.

So, just remove it.

Thanks to Russ Allbery for the original problem report.

Change-Id: I7868c8ddade2df626f5ecae597144641dfc551b7
Reviewed-on: http://gerrit.openafs.org/2838
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>