boards: thingy53_nrf5340: Reference usbd node from the common dts file

The thingy53_nrf5340 board definition based on the nrf5340dk_nrf5340
and thus added separate references to the usbd node for the secure and
non-secure targets, what unnecessary complicates things. This commit
replaces the references with one done from the common cpuapp dts file.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>