Revert the `requireCanary` check introduced in https://github.com/hashicorp/nomad/pull/8691/files#diff-1801138ac4d10f2064ba6f2e434ac9b4L430-R431 .

The change was intended to fix a case where a canary alloc may fail to
be rescheduled if all the other allocs fail as well (e.g. if all allocs
happen to be placed on a node that died).  However, it introduced some
unintended side-effects.

Reverting the change for now and will investigate further.