Remove flashloan receiver

[MathisGD]

Reasons for doing this:

• uniformisation with callbacks, both logic and interfaces
• removes one argument to the flashloan function and to the callback
• it does not seem really useful to have this flexibility, as you need to auth the initial caller in the callback function

NB: an arbitrary from in transferFrom is a critical issue for slither.

[Rubilmax]

Features lost with this change:

1. EOAs now need to trigger flash loans via callback contracts
2. Flashloans can no longer be initiated in a delegatecall context to a different receiver

[makcandrov]

I totally agree with the idea of harmonizing the callbacks. But imo the flashloan receiver could have been useful