Remove incorrect statements about account keys and session keys

[lsaether]

Brings in this open PR from the old wiki.

@burdges, any idea when the proxy key will be introduced? I'm a bit hesitant adding it to the wiki if it's not going to implemented during the Kusama cycle.

[burdges]

We've had proxy keys since March paritytech/substrate#2137 but not sure when the key took it's current form https://github.com/paritytech/substrate/blob/master/srml/democracy/src/lib.rs#L288 see also paritytech/substrate#3544