mtd: Remove mtd_erase_callback() entirely

The original purpose of mtd_erase_callback() in Linux at the time it was
imported to U-Boot, was to inform the caller that erasing is done (since
it was an asynchronous operation).

All supplied callback methods in U-Boot do nothing, but the
mtd_erase_callback() function was (until previous patch) grossly abused
in U-Boot's mtdpart implementation for completely different purpose.

Since we got rid of the abusement, remove the mtd_erase_callback()
function and the .callback member from struct erase_info entirely, in
order to avoid such problems in the future.

Signed-off-by: Marek Behún <marek.behun@nic.cz>

include/nand.h

@@ -69,7 +69,6 @@ static inline int nand_erase(struct mtd_info *info, loff_t off, size_t size)
 	instr.mtd = info;
 	instr.addr = off;
 	instr.len = size;
-	instr.callback = 0;
 
 	return mtd_erase(info, &instr);
 }