Remove redundant .c_str()s

After the tinyformat switch sprintf() family functions support passing
actual std::string objects.

Remove unnecessary c_str calls (236 of them) in logging and formatting.

src/txdb.cpp

@@ -219,7 +219,7 @@ bool CBlockTreeDB::LoadBlockIndexGuts()
                 pindexNew->nTx            = diskindex.nTx;
 
                 if (!pindexNew->CheckIndex())
-                    return error("LoadBlockIndex() : CheckIndex failed: %s", pindexNew->ToString().c_str());
+                    return error("LoadBlockIndex() : CheckIndex failed: %s", pindexNew->ToString());
 
                 pcursor->Next();
             } else {