ws: Remove the cockpit-ws --uninstalled option

While you can still run cockpit-ws from the build tree it will
spawn cockpit-session and cockpit-bridge from the $prefix.

This was already broken ... this commit just makes it official.
It was broken for two reasons:

 * cockpit-session usually runs as privileged, and cockpit-ws does
   not. It is a security hole to have cockpit-ws tell cockpit-session
   which cockpit-bridge to launch.

 * cockpit-bridge loads packages from directories based on the prefix
   and/or $XDG_DATA_DIRS, so --uninstalled didn't work here.

If someone wants to invest time in making Cockpit runnable from the
build tree, and can do so in a secure and clean way, then I wouldn't
be against it.

Closes #1385
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>