firmware-utils: remove the wndr3700 utility

It is not used anymore.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38663 3c298f89-4303-0410-b956-a3cf2f4a3e73