Remove "dav1d/" prefix from david.h includes

This partially reverts !1721 (merged). The other headers in include/dav1d do not use the dav1d/ prefix. For example, picture.h does #include "common.h" and #include "headers.h".

As far as I can tell, there's no reason for dav1d.h to use the dav1d/ prefix. Removing the prefix and using local paths is both preferable and more consistent.