samples: subsys: debug: Remove CONFIG_SEGGER_RTT assignment

This symbol does not exist. Not sure what was intended.

There's HAS_SEGER_RTT, but that symbol has no prompt, meaning .config
values have no effect on it. It is 'select'ed by the SoCs.

Remove the assignment, which is a no-op.

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