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/hardcopy.c

@@ -41,7 +41,7 @@
 #include "nvim/screen.h"
 #include "nvim/strings.h"
 #include "nvim/syntax.h"
-#include "nvim/term.h"
+#include "nvim/ui.h"
 #include "nvim/tempfile.h"
 #include "nvim/os/os.h"
 #include "nvim/os/input.h"
@@ -572,7 +572,7 @@ static void prt_message(char_u *s)
 {
   screen_fill((int)Rows - 1, (int)Rows, 0, (int)Columns, ' ', ' ', 0);
   screen_puts(s, (int)Rows - 1, 0, hl_attr(HLF_R));
-  out_flush();
+  ui_flush();
 }
 
 void ex_hardcopy(exarg_T *eap)