qm: assume correct VNC setup in 'vncproxy', disallow passwordless

qm: assume correct VNC setup in 'vncproxy', disallow passwordless

The QMP 'change' command is no longer available since QEMU 6.0, so this
cannot work - instead of replacing it, we can just remove it however.

The 'if' branch would only set the VNC socket path anew and enable
password mode, which is always set and enabled on startup already.
The 'else' branch was intended for certificate login (?), which
according to the FIXME comment is long gone anyway - simply forbid
'vncproxy' without the PVE ticket environment variable set.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>