target: Remove duplicate task completions in transport_emulate_control_cdb

This patch removes a duplicate set of transport_complete_task() calls in
target_emulate_unmap() and target_emulate_write_same() as the completion
call is already done within transport_emulate_control_cdb()

This patch also adds a check in transport_emulate_control_cdb() for the
existing SCF_EMULATE_CDB_ASYNC flag currently used by SYNCHRONIZE_CACHE
in order to handle IMMEDIATE processing.

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>