removing check_user_xattr function

This function is no longer used, the code that called
this function was removed as part of this patch:
http://review.gluster.com/#/c/3390/

Change-Id: I6e81e4b763ce302289e2be5467dc4776f0750c15
Signed-off-by: Thiago da Silva <thiago@redhat.com>
Reviewed-on: http://review.gluster.org/7336
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>