xserver-nodm-init: do not execute Xsession twice

The /etc/X11/Xserver script runs xinit /etc/X11/Xsession already so
there is no need to do /usr/bin/xinit /etc/X11/Xsession --
/etc/X11/Xserver. Having xinit nested will result in Xsession being
executed twice. To resolve this, just execute /etc/X11/Xserver directly
instead of going through xinit.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>