fix #4522: api: vncproxy: also set environment variable for ticket without websocket

fix #4522: api: vncproxy: also set environment variable for ticket without websocket

Since commit 2dc0eb61 ("qm: assume correct VNC setup in 'vncproxy',
disallow passwordless"), 'qm vncproxy' will just fail when the
LC_PVE_TICKET environment variable is not set. Since it is not only
required in combination with websocket, drop that conditional.

For the non-serial case, this was the last remaining effect of the
'websocket' parameter, so update the parameter description.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>