The IXP4xx firmware is now in linux-firmware so use these images
instead and drop the special microcode builder package.
Apply a change to the github labeler at the same time as the special
ixp4xx microcode package no longer exists after this.
Link: https://github.com/openwrt/openwrt/pull/24422
Signed-off-by: Linus Walleij <linusw@kernel.org>