icside: remove superfluous ->maskproc method

[inspired by pata_icside]

Enabling/disabling of card IRQs is handled fine by IRQ and IDE
subsystems so there is no need for custom ->maskproc method.

Moreover icside_maskproc() would enable IRQ only if it was already
enabled [because of 'if (state->enabled && !mask)' check].

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>