ipq806x: drop partitial supported boards

There are only artifacts for these boards in our tree and not even
partial support.

Drop teh stale files.

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

package/firmware/ipq-wifi/Makefile

@@ -13,7 +13,7 @@ endef
 define Build/Compile
 endef
 
-ALLWIFIBOARDS:=rt-ac58u fritz4040 nbg6617
+ALLWIFIBOARDS:=fritz4040
 ALLWIFIPACKAGES:=$(foreach BOARD,$(ALLWIFIBOARDS),ipq-wifi-$(BOARD))
 
 define Package/ipq-wifi-default
@@ -47,8 +47,6 @@ Don't install it for any other device!
   PREV_BOARD+=ipq-wifi-$(1)
 endef
 
-$(eval $(call generate-ipq-wifi-package,rt-ac58u,board-rt-ac58u.bin,ASUS RT-AC58U/RT-ACRH13))
 $(eval $(call generate-ipq-wifi-package,fritz4040,board-fritz4040.bin,AVM FRITZBox 4040))
-$(eval $(call generate-ipq-wifi-package,nbg6617,board-nbg6617.bin,ZyXEL NBG6617))
 
 $(foreach PACKAGE,$(ALLWIFIPACKAGES),$(eval $(call BuildPackage,$(PACKAGE))))