target: remove target_core_configfs.h

The remaining defintions are private to the target core and can be merged
into target_core_internal.h.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>