This answer is courtesy of Colin Donovan, vice president for theology at EWTN:
No one has been de-canonized, just de-listed. The Church reduced the number on the General Calendar used everywhere in the Roman rite. With few exceptions, the names removed from the General Calendar remain listed in the Roman Martyrology, the liturgical book which is the list of officially recognized saints to whom liturgical veneration may be made (e.g., votive Masses and Divine Office).
A very small number of saints from the early centuries were removed altogether because they were poorly attested to historically (e.g. Christopher). Since they never were formally canonized anyway, only accepted by bishops in a region, then, over time, became of wider interest, there is no question of them being de-canonized.

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