Fix #1591 - timeshift: fix mutex deadlock affecting on-demand mode.

Note: there is still an issue I've realised in that writer_flush() could
potentially result in a single packet being written out of order as two
threads have the potential to write. This isn't fatal and is probably
rare enough to ignore at this stage.

src/timeshift/timeshift_reader.c

@@ -777,7 +777,6 @@ void *timeshift_reader ( void *p )
     /* Flush unwanted */
     } else if (ts->ondemand && cur_file) {
       pthread_mutex_lock(&ts->rdwr_mutex);
-      timeshift_writer_flush(ts);
       timeshift_filemgr_flush(ts, cur_file);
       pthread_mutex_unlock(&ts->rdwr_mutex);
     }