KVM: mmu: remove uninteresting MMU "new_cr3" callbacks

The new_cr3 MMU callback has been a wrapper for mmu_free_roots since commit
e676505a (KVM: MMU: Force cr3 reload with two dimensional paging on mov
cr3 emulation, 2012-07-08).

The commit message mentioned that "mmu_free_roots() is somewhat of an overkill,
but fixing that is more complicated and will be done after this minimal fix".
One year has passed, and no one really felt the need to do a different fix.
Wrap the call with a kvm_mmu_new_cr3 function for clarity, but remove the
callback.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>