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.
Merge pull request #933 from wking/no-static-for-pictures

samples: Remove EXTRA_PATH_METADATA entries for pictures