[libpng] Update to libpng version 1.6.52

* Merge libpng version 1.6.52: tag "891defaba68f9f035dbe5b6b4b20471274160029"
* Delete all "deleted by us" files
* Update version in pnglibconf.h header comment

Change-Id: Ib900c9fe9aecfc608155991d5a64ff81217d4b61
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/libpng/+/1442549
Reviewed-by: Ina Huh <ihuh@google.com>