log: Enforce trailing newline, Remove redundant m_started_new_line

All log lines already have a trailing newline, but enforcing it allows
to delete unused code.

src/logging.h

@@ -105,13 +105,6 @@ namespace BCLog {
         size_t m_cur_buffer_memusage GUARDED_BY(m_cs){0};
         size_t m_buffer_lines_discarded GUARDED_BY(m_cs){0};
 
-        /**
-         * m_started_new_line is a state variable that will suppress printing of
-         * the timestamp when multiple calls are made that don't end in a
-         * newline.
-         */
-        std::atomic_bool m_started_new_line{true};
-
         //! Category-specific log level. Overrides `m_log_level`.
         std::unordered_map<LogFlags, Level> m_category_log_levels GUARDED_BY(m_cs);
 
@@ -253,7 +246,6 @@ inline void LogPrintFormatInternal(std::string_view logging_function, std::strin
         try {
             log_msg = tfm::format(fmt, args...);
         } catch (tinyformat::format_error& fmterr) {
-            /* Original format string will have newline so don't add one here */
             log_msg = "Error \"" + std::string{fmterr.what()} + "\" while formatting log message: " + fmt.fmt;
         }
         LogInstance().LogPrintStr(log_msg, logging_function, source_file, source_line, flag, level);