Fix handling of mouse clicks on tab-bar buttons

Fix handling of mouse clicks on tab-bar buttons

* src/xdisp.c (note_mouse_highlight): Don't attempt to highlight
tab-bar buttons.
(note_tab_bar_highlight): Function deleted: it had no effect on
display of tab-bar buttons.
(tab_bar_item_info): Mention all arguments in the commentary.
(get_tab_bar_item): Don't pay attention to mouse-highlight
information; instead, compare the button's index with the one
recorded in f->last_tab_bar_item.
(handle_tab_bar_click): Don't attempt to show tab-bar buttons in
pressed or released state: that isn't supported.  Determine
whether to generate a tab-bar button click based on DOWN_P
argument, not on mouse-highlight, which has no effect on tab-bar
display.  (Bug#47581)