PlaneShell: Remove broken optimization for Trapezoid

PlaneShell's SafetyToIn and SafetyToOut contained what looks like an optimization for the case N == 4. However, the condition / branches were swapped so that the code would use the general loop for N == 4 and assume exactly four planes for N != 4. This wasn't a problem in practice because VecGeom only ever uses PlaneShell with N == 4, for the Trapezoid.

Performance measurements show that the re-ordering of Max operations yields a very minor improvement of at most 1.5% - 2%. As it wasn't active up to now, just remove the code path. As such, no performance difference expected nor measured.

(cherry picked from commit d695d4bb)