boards: silabs: efr32_radio: Remove duplicate usart0 definition

In the <board>.dts file, the definition for usart0 was found to be
redundant as the same information is already provided in the included
.dtsi file. this commit removes the duplicate definition of usart0,
resulting in a cleaner and more maintainable device tree configuration.

Signed-off-by: Arunmani Alagarsamy <arunmani.a@capgemini.com>