iscsi: Remove pointless runtime check of macro value

raw_bsd already has QEMU_BUILD_BUG_ON(BDRV_SECTOR_SIZE != 512), so iscsi
should relax.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
