[6.x] Update Mitigation by Armor

The needed creature_classlevelstats.basearmor  will be submitted via issue.

Combat Rating ArmorPenetration is removed, because there is no character combat rating Armor Penetration since 4.0.1.

Formula was proofed and calculated in issue
#13818 
how about using the usual "= 0;" syntax ? it should make things more clear.