target: need_to_release is always false, remove redundant check and kfree

target: need_to_release is always false, remove redundant check and kfree

The removal the #ifdef'd code on CONFIG_ARCH_HAS_SG_CHAIN in
commit 02c4de53add1b79 ("target/rd: always chain S/G list")
removed the setting of need_to_release to true, so we are now left
with need_to_release always being false and hence the conditional
kfree is never reached.  Clean up the dead code by removing need_to_release
and the conditional kfree.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>