arm64: Remove ARCH_VULCAN

Commit a314520d ("arm64: disable Broadcom Vulcan platform")
did not remove the ARCH_VULCAN configuration symbol, as there were still
references to this symbol.

As of commits 240d3d5b ("gpio: xlp: update GPIO_XLP dependency") and
f85a543e ("arm64: defconfig: drop ARCH_VULCAN"), the last users
of ARCH_VULCAN have been removed.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/9e8fef2cf4f2d5648e87076bc96601cff945ce40.1641996361.git.geert+renesas@glider.be'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm64/Kconfig.platforms

@@ -309,9 +309,6 @@ config ARCH_VISCONTI
 	help
 	  This enables support for Toshiba Visconti SoCs Family.
 
-config ARCH_VULCAN
-	def_bool n
-
 config ARCH_XGENE
 	bool "AppliedMicro X-Gene SOC Family"
 	help