arm64: defconfig: drop ARCH_VULCAN

ARCH_VULCAN (Broadcom Vulcan) has been discontinued and all its
dependencies have been moved to ARCH_THUNDER2. It can be dropped
from defconfig now.

Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
Signed-off-by: Olof Johansson <olof@lixom.net>