taskqueue_enqueue_fast is an alias of taskqueue_enqueue. It is not
needed to _fast version when in interrupt context anymore. Remove all
consumers of taskqueue_enqueue_fast and remove it from taskqueue.h.
But leave the implementation in kern/subr_taskqueue.c for back-
compability of binary drivers.
Also update man page to remove the related description on
taskqueue_enqueue_fast.
Point By: jhb