ARM: configs: at91: remove CONFIG_MICROCHIP_PIT64B

Since commit f611af4c ("ARM: at91: Kconfig: implement PIT64B selection")
there is no need to explicitly select PIT64B.

Fixes: f611af4c ("ARM: at91: Kconfig: implement PIT64B selection")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220826083927.3107272-2-claudiu.beznea@microchip.com