libdvdcss: drop internal libdvdcss tree

libdvdcss: drop internal libdvdcss tree

The internal libdvdcss version was only compiled if you used internal
libdvdread too; and libdvdread was not included in the sources, so
that'd only happen if you manually added libdvdread in the build
tree. Keeping libdvdcss for that case probably isn't worth it, so
delete it from the tree. The build system part is still there, so an
internal build is possible if you add the directory back.