Geo2G4SolidFactory: minor simplification of the code

Vakhtang Tsulaiarequested to merge
tsulaia/athena:main-geo2g4 into main

Switched the code fragments dealing with Pcon's and Pgon's to the updated API from GeoModel 6.9.0.

This is a follow-up on the discussion that was given a somewhat low priority but was not forgotten.

CC @jchapman

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