codespell: Prune ignore list
Remove entries from the codespell ignore list that do not seem to be
present any more. Also, remove a deliberate typo and instead add a local
annotation to ignore it.
This list is still uncomfortably long, but unfortunately current
versions of codespell do not offer any granularity in between annotating
individual uses (which is not practical in most cases) or globally
ignoring words (which is overly broad and risks missing real errors).