Revert "lavf/concatdec: define "ffcat" and "ffconcat" extensions"

This reverts commit 6cc12353a88dfb21c4c5a38e6c83280483b0ec88.

Conflicts:
	libavformat/version.h

Allowing to automatically select the concat demuxer raises
security concerns, as it allows a possibly hostile file to
access any file on the system. Guessing the format based on
the file name extension does not allow to enable the safe
mode designed to avoid it.
