logging: remove unused return value from LogPrintStr

`LogPrintStr` returns the number of characters printed. This number is
doubled if both logging to console and logging to file is enabled. As
the return value is never used, I've opted to remove it instead of try
to fix it.

Credit: @laanwj

src/logging.h

@@ -86,7 +86,7 @@ namespace BCLog {
         std::atomic<bool> m_reopen_file{false};
 
         /** Send a string to the log output */
-        int LogPrintStr(const std::string &str);
+        void LogPrintStr(const std::string &str);
 
         /** Returns whether logs will be written to any output */
         bool Enabled() const { return m_print_to_console || m_print_to_file; }