feat(ui): restore has('gui_running')

Problem:
has('gui_running') is still common in the wild and our answer has
changed over time, causing frustration.
95a6ccbe9f

Solution:
Use stdin_tty/stdout_tty to decide if a UI is (not) a GUI.

src/nvim/ui.h

@@ -107,7 +107,6 @@ struct ui_t {
   char *term_name;
   char *term_background;
   int term_colors;
-  int stdin_fd;
   bool stdin_tty;
   bool stdout_tty;