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/ex_cmds.lua

@@ -765,11 +765,6 @@ return {
     flags=bit.bor(EXTRA, BANG, EDITCMD, ARGOPT, TRLBAR),
     func='ex_rewind',
   },
-  {
-    command='fixdel',
-    flags=bit.bor(TRLBAR, CMDWIN),
-    func='do_fixdel',
-  },
   {
     command='fold',
     flags=bit.bor(RANGE, WHOLEFOLD, TRLBAR, SBOXOK, CMDWIN),