[pallas] `pallas_call` no longer accepts `backend=`
This argument is redundant in the presence of `compiler_params=`. It made sense
when we allowed `compiler_params=` to be a multi-backend dict, but this is no
longer the case, and `backend` is always equal to `compiler_params.BACKEND`.
We expect this change to only affect a minority of Pallas users, since passing
`backend` was only really necessary to switch between Triton and Mosaic GPU,
and even then only when `compiler_params` was missing.
PiperOrigin-RevId: 870873908
