refactor: Remove redundant c_str() calls in formatting

Our formatter, tinyformat, *never* needs `c_str()` for strings.
Remove redundant `c_str()` calls for:

- `strprintf`
- `LogPrintf`
- `tfm::format`

src/policy/fees.cpp

@@ -517,7 +517,7 @@ void CBlockPolicyEstimator::processTransaction(const CTxMemPoolEntry& entry, boo
     uint256 hash = entry.GetTx().GetHash();
     if (mapMemPoolTxs.count(hash)) {
         LogPrint(BCLog::ESTIMATEFEE, "Blockpolicy error mempool tx %s already being tracked\n",
-                 hash.ToString().c_str());
+                 hash.ToString());
         return;
     }