Explicitly setting `DEPLOY_DIR_IMAGE` is not needed, since we have
`TEMPLATECONF`.
Setting `TEMPLATECONF` on the `source` step makes both `bitbake` and
`runqemu` steps share the same `DISTRO` setting, which sets
`DEPLOY_DIR_IMAGE` properly. Remove it from readme and conf-notes shown
after sourcing the env.

Signed-off-by: Wojciech Zmuda <wojciech.zmuda@huawei.com>