Don't continue to query block-size if disk doesn't have it.

Stops poluting screen with a lot of "block-size: exception -21".

include/grub/ieee1275/ofdisk.h

@@ -22,7 +22,4 @@
 extern void grub_ofdisk_init (void);
 extern void grub_ofdisk_fini (void);
 
-extern grub_err_t grub_ofdisk_get_block_size (const char *device,
-                                              grub_uint32_t *block_size);
-
 #endif /* ! GRUB_INIT_HEADER */