Remove sigmas shield

[GreyFalcon-11737]

So why not nerf his self peel, and buff his barrier?

@GreyFalcon-11737

Sigma = 20% faster projectiles, 1000 barrier, Kinetic Grasp recharges Barrier instead of Temp HP (can overheal barrier up to 500hp), Accretion does Knockback instead of stun, no selfdamage