KVM: selftests: Drop unused kvm_userspace_memory_region_find() helper

Drop kvm_userspace_memory_region_find(), it's unused and a terrible API
(probably why it's unused).  If anything outside of kvm_util.c needs to
get at the memslot, userspace_mem_region_find() can be exposed to give
others full access to all memory region/slot information.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20231027182217.3615211-25-seanjc@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>