Remove unuseful "Connection Type" page in assistant

Also:
- Remove ambiguous kind_of_net variable from schemas
- Fix a bug with kind_of_net (user sets kind_of_net from assistant,
  modifies bandwidth afterwards in preferences, but H323 bandwidth,
  which uses kind_of_net, is not updated)
- Use more appropriate values for bandwidth with H323 (depending on
  maximum_video_tx_bitrate)
- Update defaults for video size and video bandwidth
- Fix description of video/size

maximum_video_tx_bitrate and video/size variables are not set by the
assistant anymore, but set in the schemas.