nMinDiskSpace was removed in commit rABCd181505a6b0f
Also, remove incorrect doxygen comment.
See https://doxygen.bitcoincore.org/class_c_chain.html#aeb563751f7362d4308c7c2cb35b834a5
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/23381600 | core#23381600]]