gpio/omap: remove retrigger variable in gpio_irq_handler

commit 672e302e (ARM: OMAP: use edge/level handlers from generic IRQ
framework) removed retrigger support in favor of using generic IRQ
framework.  This patch cleans up some unused remnants of that removal.

Cc: Kevin Hilman <khilman@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Cousson, Benoit <b-cousson@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Tarun Kanti DebBarma <tarun.kanti@ti.com>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Tested-by: Govindraj.R <govindraj.raja@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>