Revert "[intel/portage] keyword libpng-1.6 on amd64"

This reverts commit 72e87022f688f790f7941619f11af8d1c2a6cc51.
Revert "[intel/portage] add ABI_X86="32 64" to the amd64 make.conf"

This reverts commit 95f077912d1e3fe25c8ff4002a2824d9d5243454.