Protocol 2.5 - refactor the exposure ceiling implementation against the LTV

[The-3D]

As a result of the audit and validation process, the current implementation of the exposure ceiling resulted to be vulnerable to flashloan attacks. A fix would bring too many UX issues for borrowers that want to withdraw collateral while borrowing. Given this fact, the full implementation of the exposure ceiling will be removed. The checks for the exposure ceiling will be instead moved to the case in which LTV for an asset is set to 0. This will still allow any asset to be disabled as collateral (ltv = 0) even when there are already loans in progress with that collateral (liq threshold > 0) without causing any liquidation, something that is currently impossible in aave v2 (setting LTV = 0 can at this time be circumvented using flashloans).