st/vega: Remove.

OpenVG API seems to have dwindled away.  The code
would still be interesting if we wanted to implement NV_path_rendering
but given the trend of the next gen graphics APIs, it seems
unlikely that this becomes ARB or core.

v2: Remove a few "openvg" references left, per Emil Velikov.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

v3: Update release notes.