[MIPS] eXcite: Don't set SERIAL_RM9000.

The driver has not been merged yet so selecting it results in a warning
message.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -126,7 +126,6 @@ config BASLER_EXCITE
 	select IRQ_CPU
 	select IRQ_CPU_RM7K
 	select IRQ_CPU_RM9K
-	select SERIAL_RM9000
 	select SYS_HAS_CPU_RM9000
 	select SYS_SUPPORTS_32BIT_KERNEL
 	select SYS_SUPPORTS_64BIT_KERNEL