refactor: Remove term modules and termcap options

- Removed term.c, term.h and term_defs.h
- Tests for T_* values were removed. screen.c was simplified as a
  consequence(the best strategy for drawing is implemented in the UI layer)
- Redraw functions now call ui.c functions directly. Updates are flushed with
  `ui_flush()`
- Removed all termcap options(they now return empty strings for compatibility)
- &term/&ttybuiltin options return a constant value(nvim)
- &t_Co is still available, but it mirrors t_colors directly
- Remove cursor tracking from screen.c and the `screen_start` function. Now the
  UI is expected to maintain cursor state across any call, and reset it when
  resized.
- Remove unused code

src/nvim/globals.h

@@ -376,7 +376,6 @@ EXTERN int provider_call_nesting INIT(= 0);
 EXTERN char_u hash_removed;
 
 
-EXTERN int scroll_region INIT(= FALSE);      /* term supports scroll region */
 EXTERN int t_colors INIT(= 0);              /* int value of T_CCO */
 
 /*