MINOR: task: remove tasklet_insert_into_tasklet_list()

MINOR: task: remove tasklet_insert_into_tasklet_list()

This function is only called at a single place and adds more confusion
than it removes. It also makes one think it could be used outside of
the scheduler while it must absolutely not. Let's just move its two
lines to the call place, making the code more readable there. In
addition this clearly shows that the preliminary LIST_INIT() is
useless since the entry is immediately overwritten.