ramips: TP-Link TL-WR902AC v3: don't build factory image

ramips: TP-Link TL-WR902AC v3: don't build factory image

The line that produces factory image was accidentally left by me while
testing before inital commit.

I came to the conclusion that flashing from OEM firmware does not work
(seems to share this behavior with other tplinks based on mt7628).

I have not done any further analysis, as I was unable to open the
case and attach a serial port (too much glue). Maybe i will try once
more.

So the way to do initial flashing (or un-bricking) is to use the
tftp-recover image. It is possible to revert to OEM firmware with tftp
recovery; in this case the first 512 bytes the image file need to be
cut off.

Signed-off-by: Peter Lundkvist <peter.lundkvist@gmail.com>
[add explaination provided via mail as commit message]
Signed-off-by: Mathias Kresin <dev@kresin.me>