target: simplify core_tmr_abort_task

list_for_each_entry_safe is necessary if list objects are deleted from
the list while traversing it.  Not the case here, so we can use the base
list_for_each_entry variant.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>