Comment, cast and assert cleanup in pow.cpp

Summary:
Comments improved and a superfluous assert which is
covered by subsequent code is removed

Test Plan:
make check
../qa/pull-tester/rpc-tests.py -extended

Reviewers: #bitcoin_abc, deadalnix, kyuupichan

Reviewed By: #bitcoin_abc, kyuupichan

Differential Revision: https://reviews.bitcoinabc.org/D635