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>