ath79: remove unused copy-file build step

Signed-off-by: Matthias Schiffer <[email protected]>

target/linux/ath79/image/Makefile

@@ -48,9 +48,6 @@ define Build/relocate-kernel
 	rm -rf [email protected]
 endef
 
-define Build/copy-file
-	cat "$(1)" > "$@"
-endef
 
 define Device/Default
   ATH_SOC :=