cosmetic: UDM-serial: clean up the remainders of dead driver

The following serial drivers do not exist any more.

 - ns9750_serial.c: deleted by commit 4cfc611b
 - s3c4510b_uart.c: deleted by commit afad4029
 - serial_clps7111.c: deleted by commit f2e08015
 - serial_netarm.c: deleted by commit b411eb30

This commit cleans up UDM-serial.txt.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

doc/driver-model/UDM-serial.txt

@@ -84,18 +84,10 @@ III) Analysis of in-tree drivers
   during conversion. This driver is implemented in very universal manner,
   therefore it'll be necessary to properly design it's platform_data.
 
-  ns9750_serial.c
-  ---------------
-  Unmaintained port. Code got removed.
-
   opencores_yanu.c
   ----------------
   No support for CONFIG_SERIAL_MULTI. Simple conversion possible.
 
-  s3c4510b_uart.c
-  ---------------
-  No support for CONFIG_SERIAL_MULTI. Simple conversion possible.
-
   sandbox.c
   ---------
   No support for CONFIG_SERIAL_MULTI. Simple conversion possible.
@@ -104,10 +96,6 @@ III) Analysis of in-tree drivers
   --------
   This is a complementary part of NS16550 UART driver, see above.
 
-  serial_clps7111.c
-  -----------------
-  No support for CONFIG_SERIAL_MULTI. Simple conversion possible.
-
   serial_imx.c
   ------------
   No support for CONFIG_SERIAL_MULTI. Simple conversion possible. This driver
@@ -129,10 +117,6 @@ III) Analysis of in-tree drivers
   ------------
   No support for CONFIG_SERIAL_MULTI. Simple conversion possible.
 
-  serial_netarm.c
-  ---------------
-  No support for CONFIG_SERIAL_MULTI. Simple conversion possible.
-
   serial_pl01x.c
   --------------
   No support for CONFIG_SERIAL_MULTI. Simple conversion possible, though this