ARM: defconfig: drop CONFIG_MICROCHIP_PIT64B

This is now default-enabled when CONFIG_SOC_SAMA7 is
used, so the defconfig entry can be dropped.

Fixes: f611af4c ("ARM: at91: Kconfig: implement PIT64B selection")
Acked-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>