target: remove transport_generic_remove

All callers that never have the session_reinstatement flag set can trivially
be converted to transport_put_cmd.  Opencode the session reinstatement code
in transport_generic_free_cmd, which was the only caller ever asking for it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>