drivers: serial: liteuart: Remove unused UART_LITEUART_PORT_0 symbol

Added in commit f9efca4b ("boards: riscv32: add LiteX VexRiscV
board"), then never used.

Found with a script.

Also change UART_LITEUART from a 'menuconfig' symbol to a 'config'
symbol, as it's no longer followed by symbols that depend on it
(UART_LITEUART_PORT_0 should have been a plain 'config' too).

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