Restore the "default" Formatter

Currently we don't have a "default" formatter in the 2.4 version of ImageField. This is a problem because "default" is what CCK sets all formatters to upon new field creation. I couldn't find an issue for the original decision, #318058: Image not displayed is about as close as I could find.

This patch restores the "default" formatter, removes the imagefield_update_3() update (which changed the formatter), and adds imagefield_update_4(), which sets the formatter back to default.