osd: get_primary_shard should not call pgtemp_undo_primaryfirst

In this variant of get_primary_shard, the function attempts to undo
the primaryfirst ordering.  This is incorrect, because pg_to_acting_osds
has already applied this.

The fix is simply to remove this code.

This is unlikely to be a significant problem, as this function is not
used in the critical locations.  We are building new test harnesses
which will rely on this behaviour.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>