Remove onnxsim dependency from DaCeML

We currently use `onnxsim` to simplify the ONNX graph before transforming it into an SDFG. The main benefit we currently get from this is that `onnxsim` applies constant folding, which allows us to handle much simpler cases for the supported ops, such as only having constant inputs for some transformation parameters, like the axes of a reduction.

`onnxsim` is not a reliable dependency and should be replaced with a constant folding pass that simplifies the SDFG at the same level.