Refactor CouplingMap.make_symmetric() to use rustworkx's make_symmetric()

[HedroPedro]

The minimum supported version of rustworkx is now 0.15.0, so the compatibility workaround is no longer needed. This PR replaces the Python implementation of CouplingMap.make_symmetric() with the native PyDiGraph.make_symmetric() implementation and removes the obsolete TODO.

The previous implementation manually iterated over the graph edges and added missing reverse edges. PyDiGraph.make_symmetric() implements the same behavior natively in rustworkx, so this change removes duplicate logic while relying on the optimized implementation.

Both implementations achieves the same results.

Validation realized

• tox
  The existing test suite passes without modification, indicating that the native implementation preserves the expected behavior.

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description: Chat GPT
• I used the following tool to generate or modify code:

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