nvme: Remove duplicate completion printing routine

Both nvme_dump_completion and nvme_qpair_print_completion print
completions. The latter is better. Recode the two instances of
nvme_dump_completion to use nvme_qpair_print_completion and delete the
former. No sense having two nearly identical routines. A future commit
will convert this to sbuf.

Sponsored by: Netflix
Reviewed by: chuck
Differential Revision: https://reviews.freebsd.org/D41308