Block DC, fix label and remove duplicate code in gx_distortion

[chy-causer]

Hi there,

As the title says, this patch concerns the gx_distortion, or Multi Band Distortion module:

1. Before this patch, when changing the offset, DC is output. By switching to cubicnl_nodc this is fixed.
2. There is smoothing on the slider now, to remove the clicks when changing the offset.
3. The offset was, I think, incorrectly labelled as level. I have kept the slider name "level" so as people's saved settings are preserved, but now the panel displays "Offset", with a tooltip which I copied from another module.
4. There was some redundant code as I suspect the minimum requirement for Faust has been bumped.

Speaking of Faust version, the generated code has been generated with FAUST Version 2.81.10, as packaged by Ubuntu for 26.04.

I have not touched distortion.dsp, which seems to have similar issues, but I suspect may be dead code.