target: remove the unused SCF_* flags

This patch contains a squashed version to remove unused  SCF_* flags:

target: remove the unused SCF_SE_DISABLE_ONLINE_CHECK flag
target: remove the unused SCF_CMD_PASSTHROUGH_NOALLOC flag
target: remove the unused SCF_EMULATE_SYNC_UNMAP flag
target: remove the unused SCF_EMULATE_SYNC_CACHE flag

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