Once completed, the act of canonization is irrevocable. In some cases a person has been popularly "canonized" without official solemnization by the Church . . . yet any act short of solemn canonization by the Roman pontiff is not an infallible declaration of sanctity. Should circumstances demand, the Church may limit the public cult of such a person popularly "canonized" (vol. 3, 55-56, 59, 61).
http://www.ewtn.com/library/ANSWERS/CACANONI.HTM
The removal of saints from the liturgical calendar is explained in the Second Vatican Council’s 1963 document
Constitution on the Sacred Liturgy:
111. Lest the feasts of the saints should take precedence over the feasts which commemorate the very mysteries of salvation, many of them should be left to be celebrated by a particular Church or nation or family of religious; only those should be extended to the universal Church which commemorate saints who are truly of universal importance.
Then in 1969, the
motu proprio of Paul VI for the reform of the Liturgical calendar for says,
“With the passage of centuries, it must be admitted, the faithful have become accustomed to many special religious devotions that the principal mysteries of the redemption have lost their proper place. This was due partly to the increased number of vigils, holydays, and octaves, partly to the gradual overlapping of various seasons in the liturgical year.
The purpose of the reordering of the liturgical year and of the norms accomplishing its reform, therefore, is that through faith, hope, and love the faithful may share more deeply in "the whole mystery of Christ as it unfolds throughout the year."
With the passage of centuries, it must be admitted, the faithful have become accustomed to many special religious devotions that the principal mysteries of the redemption have lost their proper place. This was due partly to the increased number of vigils, holydays, and octaves, partly to the gradual overlapping of various seasons in the liturgical year.
Undeniably, however, over the course of the centuries more feasts of the saints were introduced than was necessary; therefore the Council pointed out: "Lest the feasts of the saints take precedence over the feasts commemorating the very mysteries of salvation, many of them should be left to be celebrated by a particular Church or nation or religious family; those only should be extended to the universal Church that commemorate saints of truly universal significance."
To put these decrees of the Council into effect, the names of some saints have been deleted from the General Calendar and permission was granted to restore the memorials and veneration of other saints in those areas with which they have been traditionally associated. The removal of certain lesser-known saints from the Roman Calendar has allowed the addition of the names of martyrs from regions where the Gospel spread later in history. In consequence, the single catalogue displays in equal dignity as representatives of all peoples those who either shed their blood for Christ or were outstanding in the heroic virtues. “
http://www.catholicculture.org/docs/...fm?recnum=5934
The bottom line is this: St. Christopher (and others) remains a saint and is still deserving of our love and devotion. It also means you can keep the St. Christopher medal in your car.
Further reading:
General Norms for the Liturgical Year and the Calendar:
http://catholicculture.org/docs/doc_...fm?recnum=5932