When you are using complete build and not the imagebuilder like in the thread Build for TP-Link TL-WR841N(D) [all versions] you need to call make menuconfig and remove IPV6 by hand.
You cand find this option in menuconfig at /Global build settings/Enable IPv6 support in packages.
Check if you removed all IPv6 packages too, like /LuCI/protocols/luci-proto-ipv6.