[MNT] Drop `CUDA_USE_NVIDIA_BINDING` and always use `cuda.core` (and fallback to `cuda.bindings`)

[leofang]

Before cuda-bindings (the old cuda-python) was a thing, Numba already had ctype-based driver/runtime API bindings for many years. After cuda-python was out, there is a CUDA_USE_NVIDIA_BINDING that allows users to switch to the official bindings, but the default has always been off and needs users' explicit opt-in.

As part of the RFE #128 we should move to drop the ctypes-based bindings entirely. We always use cuda.core whenever possible, and use cuda.bindings as a fallback in case cuda.core has not caught up.