of/irq: Drop obsolete 'interrupts' vs 'interrupts-extended' text

a9ecdc0fdc54 ("of/irq: Fix lookup to use 'interrupts-extended' property
first") updated the description to say that:

  - Both 'interrupts' and 'interrupts-extended' may be present
  - Software should prefer 'interrupts-extended'
  - Software that doesn't comprehend 'interrupts-extended' may use
    'interrupts'

But there is still a paragraph at the end that prohibits having both and
says 'interrupts' should be preferred.

Remove the contradictory text.

Fixes: a9ecdc0fdc54 ("of/irq: Fix lookup to use 'interrupts-extended' property first")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: stable@vger.kernel.org	# v3.13+
Acked-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>