ARM: defconfig: drop CONFIG_SERIAL_OMAP references

This driver is mutually exclusive with the new 8250_OMAP
driver, so 'make defconfig' turns it off already. Drop
the reference now.

Fixes: 077e1cde ("ARM: omap2plus_defconfig: Enable 8250_OMAP")
Fixes: f98d4514 ("ARM: multi_v7_defconfig: Enable 8250-omap serial driver and use it by default")
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>