Current in oe-core libpng's latest version is 1.6.0. For lsb test, it requires libpng.so.12. So libpng-1.2.50 is added back. As Mark Hatle's suggestion that libpng-1.2 should coexist with higher verison: We want to make sure we have both the old and new versions to meet LSB compliance (for people who have that enabled) as well as the new version for newer applications.
Fixed by http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=ce4faa00ec53df5417689065fda855c50818a990