kernel: drop CONFIG_PROC_DEVICETREE in 3.18

This option was removed in upstream commit 8357041.

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43607 3c298f89-4303-0410-b956-a3cf2f4a3e73