Remove mean() from the labels

[vincentarelbundock]

Obvious from the call, and takes space.

[vincentarelbundock]

@artiom-matvei in some contexts with comparisons() and avg_comparisons(), we get estimate labels that look like this:

mean(blue) - mean(red)

I would like to go back to the simpler display:

blue - red

This might just require changing this:

pymarginaleffects/marginaleffects/sanity.py

Line 132 in c4421f0

lab = {

But there may also be some tests to adjust.