ath79: fix build warning

ath79: fix build warning

The image build code for the Ubiquiti Nanostation AC series adds the
factory image as to be build image. The same is already done by an
included recipe which results into an expanded IMAGE variable of:

  IMAGES = sysupgrade.bin factory.bin factory.bin

The build system doesn't like these duplicates and issues the following
warning:

  Makefile:82: warning: overriding recipe for target...

Get remove the duplicate factory image to get rid of the warning.

Fixes: 5736af8024be ("ath79: Add support for Ubiquiti NanoStation AC loco")
       fa3c2676ab0c ("ath79: Add support for Ubiquiti Nanostation AC")

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