Revert "KVM: x86/mmu: Zap only the relevant pages when removing a memslot"

This reverts commit 4e103134.
Alex Williamson reported regressions with device assignment with
this patch.  Even though the bug is probably elsewhere and still
latent, this is needed to fix the regression.

Fixes: 4e103134 ("KVM: x86/mmu: Zap only the relevant pages when removing a memslot", 2019-02-05)
Reported-by: Alex Willamson <alex.williamson@redhat.com>
Cc: stable@vger.kernel.org
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>