Revert "legacy: support up to libpng 1.6.3"

This reverts commit 0f44dd5d954ca3437602c62928abb6cb4c811a6c.
Revert "Support for up to libpng 1.6.3"

This reverts commit bf0d80ce83e6d39713697c7e32eeba9a6f5115a2.