KVM: x86/mmu: Don't flush TLBs when clearing Dirty bit in shadow MMU

KVM: x86/mmu: Don't flush TLBs when clearing Dirty bit in shadow MMU

Don't force a TLB flush when an SPTE update in the shadow MMU happens to
clear the Dirty bit, as KVM unconditionally flushes TLBs when enabling
dirty logging, and when clearing dirty logs, KVM flushes based on its
software structures, not the SPTEs.  I.e. the flows that care about
accurate Dirty bit information already ensure there are no stale TLB
entries.

Opportunistically drop is_dirty_spte() as mmu_spte_update() was the sole
caller.

Link: https://lore.kernel.org/r/20241011021051.1557902-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>