gpio: omap: drop omap1 mpuio specific irq_mask/unmask callbacks

Originally OMAP MPUIO GPIO irqchip was implemented using Generic irq
chip, but after set of reworks Generic irq chip code was replaced by
common OMAP GPIO implementation and finally removed by
commit d2d05c65 ("gpio: omap: Fix regression for MPUIO interrupts").
Unfortunately, above commit left .irq_mask/unmask callbacks assigned
as below for MPUIO GPIO case:
	irqc->irq_mask = irq_gc_mask_set_bit;
	irqc->irq_unmask = irq_gc_mask_clr_bit;

This now causes boot failure on OMAP1 platforms, after
commit 450fa54c ("gpio: omap: convert to use generic irq handler")
which forces these callbacks to be called during GPIO IRQs mapping
from gpiochip_irq_map:

Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = c0004000
[00000000] *pgd=00000000
Internal error: Oops: 75 [#1] ARM
Modules linked in:
CPU: 0 PID: 1 Comm: swapper Not tainted 4.4.0-rc1-e3-los_afe0c+-00002-g25379c0-dirty #1
Hardware name: Amstrad E3 (Delta)
t...