samples: Remove EXTRA_PATH_METADATA entries for pictures

I'd added them earlier to test that a configuration edit could preserve the
original output locations.  However, it is likely that you have quite a number
of static files, and we shouldn't recommend listing explicit paths for all of
them.  With this configuration change, the pictures will be copied into the
output directory using their original relative path (e.g.
`pictures/Fat_Cat.jpg` without the
`static`).  Any |filename|-style links will be updated automatically.

If you *want* the pictures to end up in a `static` directory, it's easier to
just organize your source that way.