Remove unused "prefix" signaling message field
Remove unused "prefix" signaling message field

The WebUI no longer uses the "prefix" parameter, and it was never used
by the mobile apps.

The Android app declares the field in its NCSignalingMessage parcelable
object, but if a received message does not contain the field it will
be just set to an empty value.

Signed-off-by: Daniel Calviño Sánchez <bd7629e975cc0e00ea7cda4ab6a120f2ee42ac1b@gmail.com>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed