fix unused variables

[senekor]

First of all, thank you for making this public! I just finished building it and love it.

One mistake I mad was that I ordered M2 screws and inserts, because I was confused by the line

insert_diameter=3.2;

This diameter actually matches the M2 inserts. But the variable isn't even used. So I fixed it, which should prevent others from making the same mistake as me and allow the advertised customization.

(the variable insert_height was also unused)

Btw. is there any way to tip you? Have you considered setting up a GitHub sponsor thingy?