KVM: x86/mmu: Remove KVM's MMU shrinker

Remove KVM's MMU shrinker and (almost) all of its related code, as the
current implementation is very disruptive to VMs (if it ever runs),
without providing any meaningful benefit[1].

Alternatively, KVM could repurpose its shrinker, e.g. to reclaim pages
from the per-vCPU caches[2], but given that no one has complained about
lack of TDP MMU support for the shrinker in the 3+ years since the TDP MMU
was enabled by default, it's safe to say that there is likely no real use
case for initiating reclaim of KVM's page tables from the shrinker.

And while clever/cute, reclaiming the per-vCPU caches doesn't scale the
same way that reclaiming in-use page table pages does.  E.g. the amount of
memory being used by a VM doesn't always directly correlate with the
number vCPUs, and even when it does, reclaiming a few pages from per-vCPU
caches likely won't make much of a dent in the VM's total memory usage,
especially for VMs with huge amounts of memory.

Lastly, if it turns out that there is a strong use case for dropping the
per-vCPU caches, re-introducing the shrinker registration is trivial
compared to the complexity of actually reclaiming pages from the caches.

[1] https://lore.kernel.org/lkml/Y45dldZnI6OIf+a5@google.com
[2] https://lore.kernel.org/kvm/20241004195540.210396-3-vipinsh@google.com

Suggested-by: Sean Christopherson <seanjc@google.com>
Suggested-by: David Matlack <dmatlack@google.com>
Signed-off-by: Vipin Sharma <vipinsh@google.com>
Link: https://lore.kernel.org/r/20241101201437.1604321-2-vipinsh@google.com
[sean: keep zapped_obsolete_pages for now, massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>

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