Remove redundant sentence in ActivateBestChain comment

Summary:
This concludes backport of core#20749 [16/16]
https://github.com/bitcoin/bitcoin/pull/20749/commits/67c9a83df19c6e2a2edb32336879204e7770b4a7

Test Plan: NA. Trivial documentation change.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11187