serial: doc: Un-document non-existing uart_write_console()

uart_write_console() never existed, not even when the "new
uart_write_console function" was documented.

Fixes: 67ab7f59 ("[SERIAL] Update serial driver documentation")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>