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>