KVM: arm64: Don't zero VTTBR in __tlb_switch_to_host()

HCR_EL2.TGE=0 is sufficient to disable stage-2 translation, so there's
no need to explicitly zero VTTBR_EL2.

Link: https://lore.kernel.org/r/20231018233212.2888027-2-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>