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>
diff --git a/doc/driver-model/serial-howto.txt b/doc/driver-model/serial-howto.txt
index 76ad629..4706d56 100644
--- a/doc/driver-model/serial-howto.txt
+++ b/doc/driver-model/serial-howto.txt
@@ -4,8 +4,6 @@
 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