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/memfile.c

@@ -954,7 +954,6 @@ static void mf_do_open(memfile_T *mfp, char_u *fname, int flags)
 #ifdef HAVE_SELINUX
     mch_copy_sec(fname, mfp->mf_fname);
 #endif
-    mch_hide(mfp->mf_fname);        // try setting the 'hidden' flag
   }
 }