Replace init_backend with ggml_backend_init_best

[WhyNotHugo]

ggml now provides a function to initialise the best available backend. Use that instead of duplicating similar logic locally.

[wbruna]

Looks like #1184 would supersede this.

[WhyNotHugo]

My goal with this (and a few follow-up PRs) is to allow compiling stable-diffusion in a way that's usable with ggml compiled with GGML_BACKEND_DL, so that it can be shipped with distribution packages which ship separate packages for each backend.

For this, I'd avoid the backend-specific functions (e.g.: ggml_backend_cpu_init) and replace them with calls such as ggml_backend_dev_by_name("CPU").