Refactor Mana Shield damage handling

[kphoenix137]

Moves reduction to Mana while Mana Shield is active into a Player member function ApplyManaShieldToDamage(), and removes GetManaShieldDamageReduction().

ApplyManaShieldToDamage() takes totalDamage as an argument and reduces the Player's Mana (if applicable), and returns the remaining amount of damage to be dealt to the Player's Life.