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/bitcoind.cpp

@@ -70,7 +70,7 @@ static bool AppInit(int argc, char* argv[])
             strUsage += "\n" + gArgs.GetHelpMessage();
         }
 
-        tfm::format(std::cout, "%s", strUsage.c_str());
+        tfm::format(std::cout, "%s", strUsage);
         return true;
     }