This reverts commit d219f899f4b2fb4b39ebc1dff9fb648fc5d6d112.
API change: Un-deprecate font kern API.
Part of https://github.com/harfbuzz/harfbuzz/issues/1682
We should document that this API is only necessary to hook up
non-OpenType kerning. hb-ot-font will continue to NOT implement them.