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/msgpack_rpc/channel.c

@@ -24,7 +24,6 @@
 #include "nvim/memory.h"
 #include "nvim/os_unix.h"
 #include "nvim/message.h"
-#include "nvim/term.h"
 #include "nvim/map.h"
 #include "nvim/log.h"
 #include "nvim/misc1.h"
@@ -407,8 +406,8 @@ static void parse_msgpack(RStream *rstream, void *data, bool eof)
   }
 
   if (result == MSGPACK_UNPACK_NOMEM_ERROR) {
-    OUT_STR(e_outofmem);
-    out_char('\n');
+    mch_errmsg(e_outofmem);
+    mch_errmsg("\n");
     decref(channel);
     preserve_exit();
   }