Remove custom RSA implementation

[Velnbur]

Situation

Before creating a mechanism for the deanonymization of intruders in Coin Shuffle process, we implemented a custom RSA algorithm with predefined and gettable nonces. That created a security problem for users of that implementation, so we decided to move to another approach (which described in #9 )

Objective

Remove the old implementation of RSA in core

Acceptance

All repos that are using core should work as expected.