configs: remove CONFIG_MTD_M25P80
    
    When commit ed22f0605d38 ("arm64: enable spi flash memory on aarch64
    too") added CONFIG_MTD_M25P80 to the configuration generation system the
    corresponding Kconfig symbol had just been dropped upstream. So drop it
    from that system too.
    
    Signed-off-by: Paul Bolle <pebolle@tiscali.nl>