aio: remove write-only jobid & kernelinfo

The jobid (which was stored in kernelinfo) was used to look up
jobs until 1ce9182407f6, where it became essentially write only.
Remove it to simplify the code and pave the way for future work
to make aio scale better.

Note this has been slated for removal "soon" for 18 years.

Suggested by: jhb
Reviewed by: kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D47583