build-sys: remove the spice-controller library

The spice-controller was a small library to let NPAPI browser plugins
communicate with the spice client. Due to usage of vala, the library
could not promise ABI stability, and was also considerer a pretty
poor implementation.

Furthermore, major browser vendors began to phase out NPAPI support in
2013, and some would like to see it gone by the end of this
year (realistically, it may not happen though).

As an alternative, remote-viewer (the first class Spice client)
learned to connect with a file of mime type application/x-virt-viewer,
as early as February 2013 with v0.5.5. I also proposed a DBus
controller, and a simpler way to pass connection details via stdin.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Victor Toso <victortoso@redhat.com>