main: remove unused argument from backend loaders
main: remove unused argument from backend loaders

There is no need to pass the backend name string to these functions
anymore.

Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>
Reviewed-by: Quentin Glidic <241385a0e41ea713327e189bce1062decf7c1eb5@sardemff7.net>
Reviewed-by: Giulio Camuffo <65adf162ac3b23aefeb12d72b964c762bc76d738@gmail.com>

krezovic/weston,sir-murray/weston,krezovic/weston,sir-murray/weston