LU-3903 osd-zfs: Remove dead property handling code

The udmu_userprop_get_str(), udmu_userprop_set_str(), and
udmu_objset_name_get() functions are all currently unused.
Therefore this code is removed from the zfs osd.

If manipulation of zfs dataset properties is needed in the
future it can be done through the existing dsl_prop_get(),
dsl_prop_set_int(), and dsl_prop_set_string() interfaces.

This patch allows Lustre to cleanly build with versions
of zfs newer than 0.6.2.

Change-Id: I15b1ef8825dcdbed1572990d230776485d627457
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-on: http://review.whamcloud.com/7685
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>