Remove unnecessary code in message highlighting in the history window

The message to be highlighted is searched via an already formatted
timestamp, and the match contains the whole timestamp with any
preceding characters (like "["). Therefore, calling backward_char on
the start iterator of the match just goes back to the previous line
and is not needed.

src/history_window.py

@@ -610,7 +610,6 @@ class HistoryWindow:
             None)
         if result is not None:
             match_start_iter, match_end_iter = result
-            match_start_iter.backward_char() # include '[' or other character before time
             match_end_iter.forward_to_tag_toggle(self.history_buffer.eol_tag)
             self.history_buffer.apply_tag_by_name('highlight', match_start_iter,
                     match_end_iter)