Remove xml/rygel-transcode-manager.xml and xml/rygel-transcoder.xml from docs.

The .vala files these are generated from appear to have been removed, so this is
breaking the "make install" portion of the build on my machine.
Merge pull request #9 from cablelabs/cbl_blong/build-fix-rygel-renderer-docs

Remove xml/rygel-transcode-manager.xml and xml/rygel-transcoder.xml from docs