target: Drop left-over internal dev attribute code

Now that backend drivers are populating their own device attributes,
go ahead and remove left-over definitions + internal attribute list
of device attributes from target_core_configfs.c code

Also update TB_CIT_SETUP(dev_attrib,...) to signal ct_attr = NULL.

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>