KVM: PPC: remove orphan declarations from kvm_ppc.h

Remove the following orphan declarations from kvm_ppc.h:

1. kvmppc_mmu_priv_switch() has been removed since
commit dd9ebf1f ("KVM: PPC: e500: Add shadow PID
support").
2. kvmppc_core_destroy_mmu() has been removed since
commit ecc0981f ("KVM: ppc: cosmetic changes to mmu
hook names").
3. kvmppc_prepare_vrma() has been removed since
commit aa04b4cc

 ("KVM: PPC: Allocate RMAs (Real Mode
Areas) at boot for use by guests").

So remove the declarations for them from header file.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220913075029.682327-10-cuigaosheng1@huawei.com