libpng system package is broken

[springmeyer]

Currently, trying to build the libpng package gives:

$ ./mason build libpng system
 The MASON_PKGCONFIG_FILE variable not found in script.sh. Please either provide this variable or override the mason_cflags function hook

This is likely fallout that went unnoticed until now from #356.

We could fix by following the lead of the zlib system package. Or we could remove this package since I don't think libpng system is used.