[LUR-21] Remove the MultiFrameTrait trait

[huitseeker]

In the past, in #642, #644, #709, we introduced a MultiFrameTrait that abstracted over the differences in creating evaluation frames, and deducing a nova StepCircuit from them as a way to abstract over the differences in proving between Lurk Alpha and LEM.

We have adopted LEM and retired the Lurk alpha implementation in #873 #875.

So we only have one implementation of this trait today:

huitseeker@lightning.local»lurk-rs(main✗)» rg 'impl.*?MultiFrameTrait.*?for '
src/lem/multiframe.rs
387:impl<'a, F: LurkField, C: Coprocessor<F> + 'a> MultiFrameTrait<'a, F, C> for MultiFrame<'a, F, C> {

This trait should be removed, its implementation inlined in an impl block, and its only instance inlined for the structs that have an M: MultiFrameTrait<'a, F, C> as a generic parameter.

_LUR-21