Simplify, drop --enable-fallback-shell from configure script

Recommend using `notty` option in tty stanza instead.  See the updated
docs for details.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/tty.h

@@ -51,7 +51,6 @@ int	 tty_parse_args   (char *cmdline, struct tty *tty);
 
 int	 tty_exists	  (char *dev);
 int	 tty_exec	  (svc_t *tty);
-int	 tty_fallback	  (char *file);
 
 #endif /* FINIT_TTY_H_ */