[scripts/downloads.pl] remove broken mirrors for linux kernel

ftp.de.kernel.org / ftp.fr.kernel.org don't resolv anymore,
ftp.all.kernel.org is DNS round robin anyway

Signed-off-by: Mirko Vogt <mirko@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37323 3c298f89-4303-0410-b956-a3cf2f4a3e73