nvme: gc nvme_ctrlr_post_failed_request and related task stuff

In 4b977e6dda92 we removed the call to nvme_ctrlr_post_failed_request
because we can now directly fail requests in this context since we're in
the reset task already. No need to queue it. I left it in place against
future need, but it's been two years and no panics have resulted. Since
the static analysis (code checking) and the dyanmic analysis (surviving
in the field for 2 years, including at $WORK where we know we've gone
through this path when we've failed drives) both signal that it's not
really needed, go ahead and GC it. If we discover at a later date a flaw
in this analysis, we can add it back easily enough by reverting this and
4b977e6dda92.

Sponsored by: Netflix
Reviewed by: chuck, gallatin, jhb
Differential Revision: https://reviews.freebsd.org/D42048