Don't handle nir_intrinsic_load_kernel_input in several backends

Since !27783 (merged) landed, nir+clover is gone, and rusticl lowers nir_intrinsic_load_kernel_input in the frontend. This means that most driver backends don't need to handle nir_intrinsic_load_kernel_input and we can remove some dead code.

I left brw_kernel.c alone because it seems to still use this lowering. I also left the handling in several nir passes intact, because it's difficult to tell if it's still needed.

So far, I haven't really tested this beyond checking that it still builds.