Fix #124623: Strips are not deselected as expected

With 2.7x keymap, clicking on selected strips does not de-select others.

Seems to be caused by leftover code, when some selection functionality
was moved to `SEQUENCER_OT_select_handle`

Pull Request: https://projects.blender.org/blender/blender/pulls/124779

source/blender/editors/space_sequencer/sequencer_select.cc

@@ -1240,8 +1240,7 @@ int sequencer_select_exec(bContext *C, wmOperator *op)
 
   /* Deselect everything */
   if (deselect_all ||
-      (selection.seq1 && (extend == false && deselect == false && toggle == false) &&
-       !already_selected))
+      (selection.seq1 && (extend == false && deselect == false && toggle == false)))
   {
     changed |= ED_sequencer_deselect_all(scene);
   }