Thicknesses definitions in BarrelPlanarMPGDTracker_geo

Okay, maybe we can delete this code altogether, because I don't think these values are used at all. I had to bring it up because it was hard to tell how correct your calculation is correct. If you agree, we can remove either in this PR or in a follow up.

_Originally posted by @veprbl in https://github.com/eic/epic/pull/1014#discussion_r2739691197_

This refers to lines
https://github.com/eic/epic/blob/0963404ea572ae55d822e843b161aa3e2b6ba228/src/BarrelPlanarMPGDTracker_geo.cpp#L217-L236
which we don't believe these values are used, and I can't tell if the values calculated correctly.