Remove extra output newlines from custom.php.

All those `<?php` and `?>` with linebreaks in between create leading linebreaks at the top of all the pages output by Omeka.

For normal HTML pages this is basically harmless, but for XML, browsers tend to choke and die if there's leading whitespace.

This change just leaves the opening `<?php`. That's all you need to mark a whole file as being PHP code, so none of the linebreaks will be literally interpreted.