ipq40xx: fix boards being shown twice

Since generic images have been split to their own
Makefile boards are showing up twice in menuconfig
as $(eval $(call BuildImage)) was not dropped from
the new generic.mk.

Hence $(eval $(call BuildImage)) was being called
twice.

So, lets simply drop it from generic.mk.

Fixes: 378c7ff2

 ("ipq40xx: split generic images into own file")
Signed-off-by: Robert Marko <robert.marko@sartura.hr>

target/linux/ipq40xx/image/generic.mk

@@ -805,5 +805,3 @@ define Device/zyxel_wre6606
 	DEVICE_PACKAGES := -kmod-ath10k-ct kmod-ath10k-ct-smallbuffers
 endef
 TARGET_DEVICES += zyxel_wre6606
-
-$(eval $(call BuildImage))