ramips: drop rt-n56u factory image

The factory image has an uImage header covering the entire image and
not only the kernel. The original uImage header which covers only the
kernel is appended to the end of the image.

During LEDE boot the uImage rootfs splitter skips the whole filesystem,
can't find a valid filesystem magic and panics.

The last known working version was OpenWrt 14.07, which simply kept on
searching for an uImage header if the first found didn't resulted into
a working rootfs. This behaviour is kind of error prone since it could
produce false positives.

Since the sysupgrade image works fine in combination with the tftp
recovery for doing the initial installation of LEDE, simply drop the
factory image.

Related: FS#462

Signed-off-by: Mathias Kresin <dev@kresin.me>