Removed symlinks for dead branches on FTP (Firefox only)

Since the Firefox 2.0 days we've had "latest-X.Y" symlinks on FTP for all major versions of Firefox. With rapid release, this has quickly caused an explosion in the number of them, cluttering things up. In bug 689936 I removed all of the ones for dead branches (2.0, 3.0, 3.5), and also all for rapid releases (4.0, 5.0, 6.0, 7.0). From now on, there will be no new branch based symlinks, simply a "latest" symlink that points to the latest rapid release.