The abstract_type parameter was needed by the old IMR-based atomic-cell
serialization format. When we reverted to the simpler managed_bytes
representation, the parameter became a no-op but was never removed.
Drop it from all overloads of atomic_cell_view::from_bytes() and
atomic_cell_mutable_view::from_bytes() and update all call sites.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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