Remove set_trim_size from Circuit

[vlopes11]

This method give the misleading impression to the API consumer that plonk needs to mutate the trim size of a circuit, and that is false.

Even for variable size circuits, the implementor need to provide only a correct get_trim_size to PLONK calculate the correct trim for the public parameters, and this is the only requirement.

Proposal: remove Circuit::set_trim_size