CLEANUP: tasks: rename task_remove_from_tasklet_list() to tasklet_remove_*

CLEANUP: tasks: rename task_remove_from_tasklet_list() to tasklet_remove_*

The function really only operates on tasklets, its arguments are always
tasklets cast as tasks to match the function's type, to be cast back to
a struct tasklet. Let's rename it to tasklet_remove_from_tasklet_list(),
take a struct tasklet, and get rid of the undesired task casts.