cmd/dist: remove openbsd/mips64 from incomplete ports

Remove openbsd/mips64 from incomplete ports lists - all of the necessary code
has landed and we want to run tests so we can see/deal with remaining failures.

Update #40995

Change-Id: I5d4f89af82ff3abe57570a9a8abf889498093d32
Reviewed-on: https://go-review.googlesource.com/c/go/+/267606


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>