Cleanup of tBTCv2 rewards calculation code (#3399)

The rewards calculation for tBTCv2 was moved to
https://github.com/threshold-network/merkle-distribution repo and become
a part of all Threshold rewards calculation, ie PRE bonus, PRE ongoing,
and tBTCv2. All the reviews and new PRs around Threshold rewards should
be opened there, so we do not duplicate code.

Author: Beau Shinkle