Shield effect subtype consolidation

1. Limit "shield" effect subtype to effects with a limited damage absorption amount (magical shields affected by getShieldAmount() etc, Psi Damage Shield, and Resonance Shield). Removed subtype from many effects that are not impacted by getShieldAmount, getShieldDuration, or that grant other types of protection, such as damage redirection, damage negation, negative life, and so on.
2. Clarify that only one shield is removed and chosen at random for Shattering Blow, Sunder Armour and Impale.

See merge request !754