MAINTAINERS: KVM: arm/arm64: Remove myself as maintainer

MAINTAINERS: KVM: arm/arm64: Remove myself as maintainer

I no longer have time to actively review patches and manage the tree and
it's time to make that official.

Huge thanks to the incredible Linux community and all the contributors
who have put up with me over the past years.

I also take this opportunity to remove the website link to the Columbia
web page, as that information is no longer up to date and I don't know
who manages that anymore.

Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>