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>