stdio: Get rid of dead code, i.e. stdio_deregister()

Nobody is using stdio_deregister(), remove for good.

Note, even its parameters are not consistent with stdio_register().
So, if anyone want to introduce this again, better with some consistency.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

include/stdio_dev.h

@@ -83,7 +83,6 @@ int stdio_add_devices(void);
 int stdio_init(void);
 
 void stdio_print_current_devices(void);
-int stdio_deregister(const char *devname, int force);
 
 /**
  * stdio_deregister_dev() - deregister the device "devname".