taskqueue: drop unused tq_name field

taskqueue: drop unused tq_name field

tq_name was used write-only and besides it was just a pointer, so it
could point to some garbage in a temporary buffer that's gone.
This change shouldn't change KPI/KBI as struct taskqueue is private to
subr_taskqueue.c.
If we find a need for tq_name it can be resurrected at any moment.
taskqueue_create() interface is preserved for this purpose.

Suggested by:	jhb
MFC after:	10 days