powerpc/mm: remove orphan declarations from mmu_context.h

Remove the following orphan declarations from mmu_context.h:
1. switch_cop() and drop_cop() have been removed since
commit 6ff4d3e9 ("powerpc: Remove old unused icswx
based coprocessor support").

2. mm_iommu_cleanup() has been removed since
commit 4b6fad70 ("powerpc/mm/iommu, vfio/spapr: Put
pages on VFIO container shutdown").

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/20220913075029.682327-6-cuigaosheng1@huawei.com