Remove unused owned variable and stale comment in FieldSerializer::Serialize

[Copilot]

This PR addresses the linked review comment only: FieldSerializer::Serialize fetched owned without using it, and the adjacent comment no longer matched the implementation.
The change removes the dead local and aligns the comment with permutation-driven serialization.

• Scope

  • Updated only src/pcms/coupler/field_serializer.h in FieldSerializer::Serialize.

• Code cleanup

  • Removed unused local:

    auto owned = layout.GetOwnedHost();

• Comment accuracy

  • Reworded the exchange-plan comment to reference only permutation[i] (the actual indexing used by the loop), removing stale owned[i] wording.

// Before
auto owned = layout.GetOwnedHost();
// The exchange plan is per DOF holder: owned[i] and permutation[i] are ...

// After
// The exchange plan is per DOF holder: permutation[i] is indexed by holder.