ARM: KVM: get rid of S2_PGD_SIZE

ARM: KVM: get rid of S2_PGD_SIZE

S2_PGD_SIZE defines the number of pages used by a stage-2 PGD
and is unused, except for a VM_BUG_ON check that missuses the
define.

As the check is very unlikely to ever triggered except in
circumstances where KVM is the least of our worries, just kill
both the define and the VM_BUG_ON check.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>
(cherry picked from commit 4db845c3d8e2f8a219e8ac48834dd4fe085e5d63)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>