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

a9ecdc0 ("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: a9ecdc0 ("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>

Documentation/devicetree/bindings/interrupt-controller/interrupts.txt

@@ -30,10 +30,6 @@ should only be used when a device has multiple interrupt parents.
   Example:
 	interrupts-extended = <&intc1 5 1>, <&intc2 1 0>;
 
-A device node may contain either "interrupts" or "interrupts-extended", but not
-both. If both properties are present, then the operating system should log an
-error and use only the data in "interrupts".
-
 2) Interrupt controller nodes
 -----------------------------