x86: remove 3.6 support

The 3.6 kernel isn't used by any of the x86 subtargets, so remove it.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39403 3c298f89-4303-0410-b956-a3cf2f4a3e73