target: remove custom hex2bin() implementation

This patch drops transport_asciihex_to_binaryhex() in favor of proper
hex2bin usage from include/linux/kernel.h:hex2bin()

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>