Casper The Finality Gadget Paper : Remove Section on Validator's Pubkey Ban Post-Withdraw

According to latest version of the Casper FFG paper ;

> Once validator ν leaves the validator set, the validator’s public key is forever forbidden from rejoining the validator set. This removes the need to handle multiple start/end dynasties for a single identifier.

However, according to the most recent implementation casper contract, validator's information are [deleted after they withdraw their deposit](https://github.com/ethereum/casper/blob/a49f302c5d989d1bd6de2a005421dea26b949a5e/casper/contracts/simple_casper.v.py#L692), allowing them to deposit once again. The above cited section seems to be an implementation specification that could be omitted from the paper.

Read the cited Casper implementation in casper/contracts/simple_casper.v.py around line 692, then locate the quoted validator public-key ban section in the Casper FFG paper. Done means the paper no longer specifies a ban that conflicts with the implementation, with the surrounding text remaining coherent.

Written by the indexing model from the issue text.