cleanup/TUI: remove old unused code #9013

- Checks for ECHOE, ICANON were left over from Vim code. We already
  reference the symbols elsewhere without checking.
- newline_on_exit, intr_char: Both are vestigial remnants of Vim 4.x,
  not implemented in Nvim. intr_char is a termios/stty feature, it's
  probably not useful because users have other ways to configure their
  terminals.

src/nvim/misc1.c

@@ -2398,10 +2398,6 @@ int get_keystroke(void)
     }
     buf[len >= buflen ? buflen - 1 : len] = NUL;
     n = utf_ptr2char(buf);
-#ifdef UNIX
-    if (n == intr_char)
-      n = ESC;
-#endif
     break;
   }
   xfree(buf);