intc/xilinx_intc: Dont lower IRQ when HIE cleared

This is a little strange. It is lowering the parent IRQ pin on input
when HIE is cleared. There is no such behaviour in the real hardware.

ISR changes based on interrupt pin state are already guarded on HIE
being set. So we can just delete this if in its entirety.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>