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();

src/validation.cpp

@@ -2043,7 +2043,7 @@ bool CChainState::FlushStateToDisk(
         fDoFullFlush = (mode == FlushStateMode::ALWAYS) || fCacheLarge || fCacheCritical || fPeriodicFlush || fFlushForPrune;
         // Write blocks and block index to disk.
         if (fDoFullFlush || fPeriodicWrite) {
-            // Depend on nMinDiskSpace to ensure we can write block index
+            // Ensure we can write block index
             if (!CheckDiskSpace(gArgs.GetBlocksDirPath())) {
                 return AbortNode(state, "Disk space is too low!", _("Disk space is too low!"));
             }