[Tizen] Don't set enable_ozone_wayland_vkb and enable_xdg_shell.

Since ozone-wayland commit aeb6a5f7 they do not have any effect anymore,
and one is expected to define whether those features are enabled or not
with environment variables.

We were already setting OZONE_WAYLAND_USE_XDG_SHELL in the systemd
service file; we now set USE_OZONE_WAYLAND_VKB as well.