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

arch/arm64/configs/defconfig

@@ -56,7 +56,6 @@ CONFIG_ARCH_THUNDER=y
 CONFIG_ARCH_THUNDER2=y
 CONFIG_ARCH_UNIPHIER=y
 CONFIG_ARCH_VEXPRESS=y
-CONFIG_ARCH_VULCAN=y
 CONFIG_ARCH_XGENE=y
 CONFIG_ARCH_ZX=y
 CONFIG_ARCH_ZYNQMP=y