boards: olimex_lora_stm32wl_devkit: Remove non-applicable LPUART pins

Remove CTS/RTS LPUART pin definitions, as these aren't exposed through
the UEXT1 connector with their accompanying Tx/Rx lines, but are available
separately as GPIO pins on the CON1 pin header.

Signed-off-by: Kenneth J. Miller <ken@miller.ec>