dt-bindings:iio:adc:remove triggers

The trigger child nodes are not necessary anymore as they are defined
directly by the driver, depending on the compatible string.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Rob Herring <robh+dt@kernel.org>
Link: https://lore.kernel.org/r/20201128222818.1910764-7-alexandre.belloni@bootlin.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>