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.

This fixes an error that looks like this when you do `make install`:

    warning: failed to load external entity "../xml/rygel-transcode-manager.xml"
    ../librygel-server-docs.xml:52: element include: XInclude error : could not load ../xml/rygel-transcode-manager.xml, and no fallback was found
    warning: failed to load external entity "../xml/rygel-transcoder.xml"
    ../librygel-server-docs.xml:53: element include: XInclude error : could not load ../xml/rygel-transcoder.xml, and no fallback was found
    Succeeded - 31 warning(s)
    /usr/bin/mkdir -p /home/blong/cvp2/build/install/share/gtk-doc/html/librygel-server
    cp -d -R ./gtkdoc/html/* /home/blong/cvp2/build/install/share/gtk-doc/html/librygel-server
    cp: cannot stat ‘./gtkdoc/html/*’: No such file or directory
    make[6]: *** [install-data-hook] Error 1
    make[6]: Leaving directory `/home/blong/cvp2/build/src/rygel/doc/reference/librygel-server'

I'm not sure why it doesn't happen on your machines.

The file I changed says that that section is generated, but I don't know where it's generated from. It may be useful to re-run whatever script generates that.