serial-howto: remove altera_jtag_uart and altera_uart from the list

Since both altera_jtag_uart and altera_uart are converted to driver
model, remove them from the list of drivers remaining to convert.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Chin Liang See <clsee@altera.com>

doc/driver-model/serial-howto.txt

@@ -4,8 +4,6 @@ How to port a serial driver to driver model
 About 16 of 33 serial drivers have been converted as at September 2015. It
 is time for maintainers to start converting over the remaining serial drivers:
 
-   altera_jtag_uart.c
-   altera_uart.c
    arm_dcc.c
    lpc32xx_hsuart.c
    mcfuart.c