[SofaPython][SofaGuiQt] REMOVE: public export of target SofaExporter

[marques-bruno]

SofaExporter was a linked to SofaPython & SofaGuiQt under the PUBLIC scope while that dependency is internal & should not be exposed publicly

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This PR:

• builds with SUCCESS for all platforms on the CI.
• does not generate new warnings.
• does not generate new unit test failures.
• does not generate new scene test failures.
• does not break API compatibility.
• is more than 1 week old (or has fast-merge label).

Reviewers will merge only if all these checks are true.

[marques-bruno]

FYI I didn't find any other PUBLIC link of SofaExporter in sofa's CMakeLists