Remove effect_ref from callback_wrapper.
Remove effect_ref from callback_wrapper.

As `effect` can be null, this can create a null `&mut` pointer, which is UB.

rust-dsp/rust-vst,rust-dsp/rust-vst,Boscop/rust-vst2,overdrivenpotato/rust-vst2,Boscop/rust-vst2,overdrivenpotato/rust-vst2