doc: Tidy up nMinDiskSpace comment

nMinDiskSpace was removed in commit
04cca33094

Also, remove incorrect doxygen comment.
See https://doxygen.bitcoincore.org/class_c_chain.html#aeb563751f7362d4308c7c2cb35b834a5

src/chain.cpp

@@ -5,9 +5,6 @@
 
 #include <chain.h>
 
-/**
- * CChain implementation
- */
 void CChain::SetTip(CBlockIndex *pindex) {
     if (pindex == nullptr) {
         vChain.clear();