Follow-up to #75932.
Since these icons are only used by the export plugin, it makes sense to
move them and generate the headers there.
The whole `detect.is_active()` logic seems to be a leftover from before
times, as far back as 1.0-stable it already wasn't used for anything.
So I'm removing it and moving the export icon generation to
`platform_methods`, where it makes more sense.
