dts/bindings: ti,tmp116: Remove unused 'alert-gpios' property

Johann Fischer pointed out that the driver for this sensor
(master/drivers/sensor/tmp116/tmp116.c) doesn't use GPIOs, in
https://github.com/zephyrproject-rtos/zephyr/pull/21605, though there
seems to be an ALERT pin from looking at the datasheet
(http://www.ti.com/lit/ds/symlink/tmp116.pdf

).

Remove the unused property declaration.

I was originally just going to change a 'category: optional' to
'required: false' (and 'type: compound' to 'type: phandle-array').
Either solution is fine with me. Could keep the declaration if people
are planning to use it soon.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>